source: mainline/kernel/generic/src/mm/as.c@ 8fe5980

/*
 * Copyright (c) 2001-2006 Jakub Jermar
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * - Redistributions of source code must retain the above copyright
 *   notice, this list of conditions and the following disclaimer.
 * - Redistributions in binary form must reproduce the above copyright
 *   notice, this list of conditions and the following disclaimer in the
 *   documentation and/or other materials provided with the distribution.
 * - The name of the author may not be used to endorse or promote products
 *   derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

/** @addtogroup genericmm
 * @{
 */

/**
 * @file
 * @brief       Address space related functions.
 *
 * This file contains address space manipulation functions.
 * Roughly speaking, this is a higher-level client of
 * Virtual Address Translation (VAT) subsystem.
 *
 * Functionality provided by this file allows one to
 * create address spaces and create, resize and share
 * address space areas.
 *
 * @see page.c
 *
 */

#include <mm/as.h>
#include <arch/mm/as.h>
#include <mm/page.h>
#include <mm/frame.h>
#include <mm/slab.h>
#include <mm/tlb.h>
#include <arch/mm/page.h>
#include <genarch/mm/page_pt.h>
#include <genarch/mm/page_ht.h>
#include <mm/asid.h>
#include <arch/mm/asid.h>
#include <preemption.h>
#include <synch/spinlock.h>
#include <synch/mutex.h>
#include <adt/list.h>
#include <adt/btree.h>
#include <proc/task.h>
#include <proc/thread.h>
#include <arch/asm.h>
#include <panic.h>
#include <debug.h>
#include <print.h>
#include <memstr.h>
#include <macros.h>
#include <arch.h>
#include <errno.h>
#include <config.h>
#include <align.h>
#include <arch/types.h>
#include <syscall/copy.h>
#include <arch/interrupt.h>

#ifdef CONFIG_VIRT_IDX_DCACHE
#include <arch/mm/cache.h>
#endif /* CONFIG_VIRT_IDX_DCACHE */

/**
 * Each architecture decides what functions will be used to carry out
 * address space operations such as creating or locking page tables.
 */
as_operations_t *as_operations = NULL;

/**
 * Slab for as_t objects.
 */
static slab_cache_t *as_slab;

/**
 * This lock serializes access to the ASID subsystem.
 * It protects:
 * - inactive_as_with_asid_head list
 * - as->asid for each as of the as_t type
 * - asids_allocated counter
 */
SPINLOCK_INITIALIZE(asidlock);

/**
 * This list contains address spaces that are not active on any
 * processor and that have valid ASID.
 */
LIST_INITIALIZE(inactive_as_with_asid_head);

/** Kernel address space. */
as_t *AS_KERNEL = NULL;

static int area_flags_to_page_flags(int);
static as_area_t *find_area_and_lock(as_t *, uintptr_t);
static bool check_area_conflicts(as_t *, uintptr_t, size_t, as_area_t *);
static void sh_info_remove_reference(share_info_t *);

static int as_constructor(void *obj, int flags)
{
        as_t *as = (as_t *) obj;
        int rc;

        link_initialize(&as->inactive_as_with_asid_link);
        mutex_initialize(&as->lock, MUTEX_PASSIVE);     
        
        rc = as_constructor_arch(as, flags);
        
        return rc;
}

static int as_destructor(void *obj)
{
        as_t *as = (as_t *) obj;

        return as_destructor_arch(as);
}

/** Initialize address space subsystem. */
void as_init(void)
{
        as_arch_init();

        as_slab = slab_cache_create("as_slab", sizeof(as_t), 0,
            as_constructor, as_destructor, SLAB_CACHE_MAGDEFERRED);
        
        AS_KERNEL = as_create(FLAG_AS_KERNEL);
        if (!AS_KERNEL)
                panic("can't create kernel address space\n");
        
}

/** Create address space.
 *
 * @param flags         Flags that influence the way in wich the address space
 *                      is created.
 */
as_t *as_create(int flags)
{
        as_t *as;

        as = (as_t *) slab_alloc(as_slab, 0);
        (void) as_create_arch(as, 0);
        
        btree_create(&as->as_area_btree);
        
        if (flags & FLAG_AS_KERNEL)
                as->asid = ASID_KERNEL;
        else
                as->asid = ASID_INVALID;
        
        atomic_set(&as->refcount, 0);
        as->cpu_refcount = 0;
#ifdef AS_PAGE_TABLE
        as->genarch.page_table = page_table_create(flags);
#else
        page_table_create(flags);
#endif

        return as;
}

/** Destroy adress space.
 *
 * When there are no tasks referencing this address space (i.e. its refcount is
 * zero), the address space can be destroyed.
 *
 * We know that we don't hold any spinlock.
 *
 * @param as            Address space to be destroyed.
 */
void as_destroy(as_t *as)
{
        ipl_t ipl;
        bool cond;
        DEADLOCK_PROBE_INIT(p_asidlock);

        ASSERT(atomic_get(&as->refcount) == 0);
        
        /*
         * Since there is no reference to this area,
         * it is safe not to lock its mutex.
         */

        /*
         * We need to avoid deadlock between TLB shootdown and asidlock.
         * We therefore try to take asid conditionally and if we don't succeed,
         * we enable interrupts and try again. This is done while preemption is
         * disabled to prevent nested context switches. We also depend on the
         * fact that so far no spinlocks are held.
         */
        preemption_disable();
        ipl = interrupts_read();
retry:
        interrupts_disable();
        if (!spinlock_trylock(&asidlock)) {
                interrupts_enable();
                DEADLOCK_PROBE(p_asidlock, DEADLOCK_THRESHOLD);
                goto retry;
        }
        preemption_enable();    /* Interrupts disabled, enable preemption */
        if (as->asid != ASID_INVALID && as != AS_KERNEL) {
                if (as != AS && as->cpu_refcount == 0)
                        list_remove(&as->inactive_as_with_asid_link);
                asid_put(as->asid);
        }
        spinlock_unlock(&asidlock);

        /*
         * Destroy address space areas of the address space.
         * The B+tree must be walked carefully because it is
         * also being destroyed.
         */     
        for (cond = true; cond; ) {
                btree_node_t *node;

                ASSERT(!list_empty(&as->as_area_btree.leaf_head));
                node = list_get_instance(as->as_area_btree.leaf_head.next,
                    btree_node_t, leaf_link);

                if ((cond = node->keys)) {
                        as_area_destroy(as, node->key[0]);
                }
        }

        btree_destroy(&as->as_area_btree);
#ifdef AS_PAGE_TABLE
        page_table_destroy(as->genarch.page_table);
#else
        page_table_destroy(NULL);
#endif

        interrupts_restore(ipl);

        slab_free(as_slab, as);
}

/** Create address space area of common attributes.
 *
 * The created address space area is added to the target address space.
 *
 * @param as            Target address space.
 * @param flags         Flags of the area memory.
 * @param size          Size of area.
 * @param base          Base address of area.
 * @param attrs         Attributes of the area.
 * @param backend       Address space area backend. NULL if no backend is used.
 * @param backend_data  NULL or a pointer to an array holding two void *.
 *
 * @return              Address space area on success or NULL on failure.
 */
as_area_t *
as_area_create(as_t *as, int flags, size_t size, uintptr_t base, int attrs,
    mem_backend_t *backend, mem_backend_data_t *backend_data)
{
        ipl_t ipl;
        as_area_t *a;
        
        if (base % PAGE_SIZE)
                return NULL;

        if (!size)
                return NULL;

        /* Writeable executable areas are not supported. */
        if ((flags & AS_AREA_EXEC) && (flags & AS_AREA_WRITE))
                return NULL;
        
        ipl = interrupts_disable();
        mutex_lock(&as->lock);
        
        if (!check_area_conflicts(as, base, size, NULL)) {
                mutex_unlock(&as->lock);
                interrupts_restore(ipl);
                return NULL;
        }
        
        a = (as_area_t *) malloc(sizeof(as_area_t), 0);

        mutex_initialize(&a->lock, MUTEX_PASSIVE);
        
        a->as = as;
        a->flags = flags;
        a->attributes = attrs;
        a->pages = SIZE2FRAMES(size);
        a->base = base;
        a->sh_info = NULL;
        a->backend = backend;
        if (backend_data)
                a->backend_data = *backend_data;
        else
                memsetb(&a->backend_data, sizeof(a->backend_data), 0);

        btree_create(&a->used_space);
        
        btree_insert(&as->as_area_btree, base, (void *) a, NULL);

        mutex_unlock(&as->lock);
        interrupts_restore(ipl);

        return a;
}

/** Find address space area and change it.
 *
 * @param as            Address space.
 * @param address       Virtual address belonging to the area to be changed.
 *                      Must be page-aligned.
 * @param size          New size of the virtual memory block starting at
 *                      address. 
 * @param flags         Flags influencing the remap operation. Currently unused.
 *
 * @return              Zero on success or a value from @ref errno.h otherwise.
 */ 
int as_area_resize(as_t *as, uintptr_t address, size_t size, int flags)
{
        as_area_t *area;
        ipl_t ipl;
        size_t pages;
        
        ipl = interrupts_disable();
        mutex_lock(&as->lock);
        
        /*
         * Locate the area.
         */
        area = find_area_and_lock(as, address);
        if (!area) {
                mutex_unlock(&as->lock);
                interrupts_restore(ipl);
                return ENOENT;
        }

        if (area->backend == &phys_backend) {
                /*
                 * Remapping of address space areas associated
                 * with memory mapped devices is not supported.
                 */
                mutex_unlock(&area->lock);
                mutex_unlock(&as->lock);
                interrupts_restore(ipl);
                return ENOTSUP;
        }
        if (area->sh_info) {
                /*
                 * Remapping of shared address space areas 
                 * is not supported.
                 */
                mutex_unlock(&area->lock);
                mutex_unlock(&as->lock);
                interrupts_restore(ipl);
                return ENOTSUP;
        }

        pages = SIZE2FRAMES((address - area->base) + size);
        if (!pages) {
                /*
                 * Zero size address space areas are not allowed.
                 */
                mutex_unlock(&area->lock);
                mutex_unlock(&as->lock);
                interrupts_restore(ipl);
                return EPERM;
        }
        
        if (pages < area->pages) {
                bool cond;
                uintptr_t start_free = area->base + pages * PAGE_SIZE;

                /*
                 * Shrinking the area.
                 * No need to check for overlaps.
                 */

                /*
                 * Start TLB shootdown sequence.
                 */
                tlb_shootdown_start(TLB_INVL_PAGES, as->asid, area->base +
                    pages * PAGE_SIZE, area->pages - pages);

                /*
                 * Remove frames belonging to used space starting from
                 * the highest addresses downwards until an overlap with
                 * the resized address space area is found. Note that this
                 * is also the right way to remove part of the used_space
                 * B+tree leaf list.
                 */             
                for (cond = true; cond;) {
                        btree_node_t *node;
                
                        ASSERT(!list_empty(&area->used_space.leaf_head));
                        node = 
                            list_get_instance(area->used_space.leaf_head.prev,
                            btree_node_t, leaf_link);
                        if ((cond = (bool) node->keys)) {
                                uintptr_t b = node->key[node->keys - 1];
                                count_t c =
                                    (count_t) node->value[node->keys - 1];
                                unsigned int i = 0;
                        
                                if (overlaps(b, c * PAGE_SIZE, area->base,
                                    pages * PAGE_SIZE)) {
                                        
                                        if (b + c * PAGE_SIZE <= start_free) {
                                                /*
                                                 * The whole interval fits
                                                 * completely in the resized
                                                 * address space area.
                                                 */
                                                break;
                                        }
                
                                        /*
                                         * Part of the interval corresponding
                                         * to b and c overlaps with the resized
                                         * address space area.
                                         */
                
                                        cond = false;   /* we are almost done */
                                        i = (start_free - b) >> PAGE_WIDTH;
                                        if (!used_space_remove(area, start_free,
                                            c - i))
                                                panic("Could not remove used "
                                                    "space.\n");
                                } else {
                                        /*
                                         * The interval of used space can be
                                         * completely removed.
                                         */
                                        if (!used_space_remove(area, b, c))
                                                panic("Could not remove used "
                                                    "space.\n");
                                }
                        
                                for (; i < c; i++) {
                                        pte_t *pte;
                        
                                        page_table_lock(as, false);
                                        pte = page_mapping_find(as, b +
                                            i * PAGE_SIZE);
                                        ASSERT(pte && PTE_VALID(pte) &&
                                            PTE_PRESENT(pte));
                                        if (area->backend &&
                                            area->backend->frame_free) {
                                                area->backend->frame_free(area,
                                                    b + i * PAGE_SIZE,
                                                    PTE_GET_FRAME(pte));
                                        }
                                        page_mapping_remove(as, b +
                                            i * PAGE_SIZE);
                                        page_table_unlock(as, false);
                                }
                        }
                }

                /*
                 * Finish TLB shootdown sequence.
                 */

                tlb_invalidate_pages(as->asid, area->base + pages * PAGE_SIZE,
                    area->pages - pages);
                /*
                 * Invalidate software translation caches (e.g. TSB on sparc64).
                 */
                as_invalidate_translation_cache(as, area->base +
                    pages * PAGE_SIZE, area->pages - pages);
                tlb_shootdown_finalize();
                
        } else {
                /*
                 * Growing the area.
                 * Check for overlaps with other address space areas.
                 */
                if (!check_area_conflicts(as, address, pages * PAGE_SIZE,
                    area)) {
                        mutex_unlock(&area->lock);
                        mutex_unlock(&as->lock);                
                        interrupts_restore(ipl);
                        return EADDRNOTAVAIL;
                }
        } 

        area->pages = pages;
        
        mutex_unlock(&area->lock);
        mutex_unlock(&as->lock);
        interrupts_restore(ipl);

        return 0;
}

/** Destroy address space area.
 *
 * @param as            Address space.
 * @param address       Address within the area to be deleted.
 *
 * @return              Zero on success or a value from @ref errno.h on failure.
 */
int as_area_destroy(as_t *as, uintptr_t address)
{
        as_area_t *area;
        uintptr_t base;
        link_t *cur;
        ipl_t ipl;

        ipl = interrupts_disable();
        mutex_lock(&as->lock);

        area = find_area_and_lock(as, address);
        if (!area) {
                mutex_unlock(&as->lock);
                interrupts_restore(ipl);
                return ENOENT;
        }

        base = area->base;

        /*
         * Start TLB shootdown sequence.
         */
        tlb_shootdown_start(TLB_INVL_PAGES, as->asid, area->base, area->pages);

        /*
         * Visit only the pages mapped by used_space B+tree.
         */
        for (cur = area->used_space.leaf_head.next;
            cur != &area->used_space.leaf_head; cur = cur->next) {
                btree_node_t *node;
                unsigned int i;
                
                node = list_get_instance(cur, btree_node_t, leaf_link);
                for (i = 0; i < node->keys; i++) {
                        uintptr_t b = node->key[i];
                        count_t j;
                        pte_t *pte;
                        
                        for (j = 0; j < (count_t) node->value[i]; j++) {
                                page_table_lock(as, false);
                                pte = page_mapping_find(as, b + j * PAGE_SIZE);
                                ASSERT(pte && PTE_VALID(pte) &&
                                    PTE_PRESENT(pte));
                                if (area->backend &&
                                    area->backend->frame_free) {
                                        area->backend->frame_free(area, b +
                                            j * PAGE_SIZE, PTE_GET_FRAME(pte));
                                }
                                page_mapping_remove(as, b + j * PAGE_SIZE);                             
                                page_table_unlock(as, false);
                        }
                }
        }

        /*
         * Finish TLB shootdown sequence.
         */

        tlb_invalidate_pages(as->asid, area->base, area->pages);
        /*
         * Invalidate potential software translation caches (e.g. TSB on
         * sparc64).
         */
        as_invalidate_translation_cache(as, area->base, area->pages);
        tlb_shootdown_finalize();
        
        btree_destroy(&area->used_space);

        area->attributes |= AS_AREA_ATTR_PARTIAL;
        
        if (area->sh_info)
                sh_info_remove_reference(area->sh_info);
                
        mutex_unlock(&area->lock);

        /*
         * Remove the empty area from address space.
         */
        btree_remove(&as->as_area_btree, base, NULL);
        
        free(area);
        
        mutex_unlock(&as->lock);
        interrupts_restore(ipl);
        return 0;
}

/** Share address space area with another or the same address space.
 *
 * Address space area mapping is shared with a new address space area.
 * If the source address space area has not been shared so far,
 * a new sh_info is created. The new address space area simply gets the
 * sh_info of the source area. The process of duplicating the
 * mapping is done through the backend share function.
 * 
 * @param src_as        Pointer to source address space.
 * @param src_base      Base address of the source address space area.
 * @param acc_size      Expected size of the source area.
 * @param dst_as        Pointer to destination address space.
 * @param dst_base      Target base address.
 * @param dst_flags_mask Destination address space area flags mask.
 *
 * @return              Zero on success or ENOENT if there is no such task or if
 *                      there is no such address space area, EPERM if there was
 *                      a problem in accepting the area or ENOMEM if there was a
 *                      problem in allocating destination address space area.
 *                      ENOTSUP is returned if the address space area backend
 *                      does not support sharing.
 */
int as_area_share(as_t *src_as, uintptr_t src_base, size_t acc_size,
    as_t *dst_as, uintptr_t dst_base, int dst_flags_mask)
{
        ipl_t ipl;
        int src_flags;
        size_t src_size;
        as_area_t *src_area, *dst_area;
        share_info_t *sh_info;
        mem_backend_t *src_backend;
        mem_backend_data_t src_backend_data;
        
        ipl = interrupts_disable();
        mutex_lock(&src_as->lock);
        src_area = find_area_and_lock(src_as, src_base);
        if (!src_area) {
                /*
                 * Could not find the source address space area.
                 */
                mutex_unlock(&src_as->lock);
                interrupts_restore(ipl);
                return ENOENT;
        }

        if (!src_area->backend || !src_area->backend->share) {
                /*
                 * There is no backend or the backend does not
                 * know how to share the area.
                 */
                mutex_unlock(&src_area->lock);
                mutex_unlock(&src_as->lock);
                interrupts_restore(ipl);
                return ENOTSUP;
        }
        
        src_size = src_area->pages * PAGE_SIZE;
        src_flags = src_area->flags;
        src_backend = src_area->backend;
        src_backend_data = src_area->backend_data;

        /* Share the cacheable flag from the original mapping */
        if (src_flags & AS_AREA_CACHEABLE)
                dst_flags_mask |= AS_AREA_CACHEABLE;

        if (src_size != acc_size ||
            (src_flags & dst_flags_mask) != dst_flags_mask) {
                mutex_unlock(&src_area->lock);
                mutex_unlock(&src_as->lock);
                interrupts_restore(ipl);
                return EPERM;
        }

        /*
         * Now we are committed to sharing the area.
         * First, prepare the area for sharing.
         * Then it will be safe to unlock it.
         */
        sh_info = src_area->sh_info;
        if (!sh_info) {
                sh_info = (share_info_t *) malloc(sizeof(share_info_t), 0);
                mutex_initialize(&sh_info->lock, MUTEX_PASSIVE);
                sh_info->refcount = 2;
                btree_create(&sh_info->pagemap);
                src_area->sh_info = sh_info;
                /*
                 * Call the backend to setup sharing.
                 */
                src_area->backend->share(src_area);
        } else {
                mutex_lock(&sh_info->lock);
                sh_info->refcount++;
                mutex_unlock(&sh_info->lock);
        }

        mutex_unlock(&src_area->lock);
        mutex_unlock(&src_as->lock);

        /*
         * Create copy of the source address space area.
         * The destination area is created with AS_AREA_ATTR_PARTIAL
         * attribute set which prevents race condition with
         * preliminary as_page_fault() calls.
         * The flags of the source area are masked against dst_flags_mask
         * to support sharing in less privileged mode.
         */
        dst_area = as_area_create(dst_as, dst_flags_mask, src_size, dst_base,
            AS_AREA_ATTR_PARTIAL, src_backend, &src_backend_data);
        if (!dst_area) {
                /*
                 * Destination address space area could not be created.
                 */
                sh_info_remove_reference(sh_info);
                
                interrupts_restore(ipl);
                return ENOMEM;
        }

        /*
         * Now the destination address space area has been
         * fully initialized. Clear the AS_AREA_ATTR_PARTIAL
         * attribute and set the sh_info.
         */     
        mutex_lock(&dst_as->lock);      
        mutex_lock(&dst_area->lock);
        dst_area->attributes &= ~AS_AREA_ATTR_PARTIAL;
        dst_area->sh_info = sh_info;
        mutex_unlock(&dst_area->lock);
        mutex_unlock(&dst_as->lock);    

        interrupts_restore(ipl);
        
        return 0;
}

/** Check access mode for address space area.
 *
 * The address space area must be locked prior to this call.
 *
 * @param area          Address space area.
 * @param access        Access mode.
 *
 * @return              False if access violates area's permissions, true
 *                      otherwise.
 */
bool as_area_check_access(as_area_t *area, pf_access_t access)
{
        int flagmap[] = {
                [PF_ACCESS_READ] = AS_AREA_READ,
                [PF_ACCESS_WRITE] = AS_AREA_WRITE,
                [PF_ACCESS_EXEC] = AS_AREA_EXEC
        };

        if (!(area->flags & flagmap[access]))
                return false;
        
        return true;
}

/** Change adress space area flags.
 *
 * The idea is to have the same data, but with a different access mode.
 * This is needed e.g. for writing code into memory and then executing it.
 * In order for this to work properly, this may copy the data
 * into private anonymous memory (unless it's already there).
 *
 * @param as            Address space.
 * @param flags         Flags of the area memory.
 * @param address       Address withing the area to be changed.
 *
 * @return              Zero on success or a value from @ref errno.h on failure.
 */
int as_area_change_flags(as_t *as, int flags, uintptr_t address)
{
        as_area_t *area;
        uintptr_t base;
        link_t *cur;
        ipl_t ipl;
        int page_flags;
        uintptr_t *old_frame;
        index_t frame_idx;
        count_t used_pages;

        /* Flags for the new memory mapping */
        page_flags = area_flags_to_page_flags(flags);

        ipl = interrupts_disable();
        mutex_lock(&as->lock);

        area = find_area_and_lock(as, address);
        if (!area) {
                mutex_unlock(&as->lock);
                interrupts_restore(ipl);
                return ENOENT;
        }

        if (area->sh_info || area->backend != &anon_backend) {
                /* Copying shared areas not supported yet */
                /* Copying non-anonymous memory not supported yet */
                mutex_unlock(&area->lock);
                mutex_unlock(&as->lock);
                interrupts_restore(ipl);
                return ENOTSUP;
        }

        base = area->base;

        /*
         * Compute total number of used pages in the used_space B+tree
         */
        used_pages = 0;

        for (cur = area->used_space.leaf_head.next;
            cur != &area->used_space.leaf_head; cur = cur->next) {
                btree_node_t *node;
                unsigned int i;
                
                node = list_get_instance(cur, btree_node_t, leaf_link);
                for (i = 0; i < node->keys; i++) {
                        used_pages += (count_t) node->value[i];
                }
        }

        /* An array for storing frame numbers */
        old_frame = malloc(used_pages * sizeof(uintptr_t), 0);

        /*
         * Start TLB shootdown sequence.
         */
        tlb_shootdown_start(TLB_INVL_PAGES, as->asid, area->base, area->pages);

        /*
         * Remove used pages from page tables and remember their frame
         * numbers.
         */
        frame_idx = 0;

        for (cur = area->used_space.leaf_head.next;
            cur != &area->used_space.leaf_head; cur = cur->next) {
                btree_node_t *node;
                unsigned int i;
                
                node = list_get_instance(cur, btree_node_t, leaf_link);
                for (i = 0; i < node->keys; i++) {
                        uintptr_t b = node->key[i];
                        count_t j;
                        pte_t *pte;
                        
                        for (j = 0; j < (count_t) node->value[i]; j++) {
                                page_table_lock(as, false);
                                pte = page_mapping_find(as, b + j * PAGE_SIZE);
                                ASSERT(pte && PTE_VALID(pte) &&
                                    PTE_PRESENT(pte));
                                old_frame[frame_idx++] = PTE_GET_FRAME(pte);

                                /* Remove old mapping */
                                page_mapping_remove(as, b + j * PAGE_SIZE);
                                page_table_unlock(as, false);
                        }
                }
        }

        /*
         * Finish TLB shootdown sequence.
         */

        tlb_invalidate_pages(as->asid, area->base, area->pages);
        /*
         * Invalidate potential software translation caches (e.g. TSB on
         * sparc64).
         */
        as_invalidate_translation_cache(as, area->base, area->pages);
        tlb_shootdown_finalize();

        /*
         * Set the new flags.
         */
        area->flags = flags;

        /*
         * Map pages back in with new flags. This step is kept separate
         * so that the memory area could not be accesed with both the old and
         * the new flags at once.
         */
        frame_idx = 0;

        for (cur = area->used_space.leaf_head.next;
            cur != &area->used_space.leaf_head; cur = cur->next) {
                btree_node_t *node;
                unsigned int i;
                
                node = list_get_instance(cur, btree_node_t, leaf_link);
                for (i = 0; i < node->keys; i++) {
                        uintptr_t b = node->key[i];
                        count_t j;
                        
                        for (j = 0; j < (count_t) node->value[i]; j++) {
                                page_table_lock(as, false);

                                /* Insert the new mapping */
                                page_mapping_insert(as, b + j * PAGE_SIZE,
                                    old_frame[frame_idx++], page_flags);

                                page_table_unlock(as, false);
                        }
                }
        }

        free(old_frame);

        mutex_unlock(&area->lock);
        mutex_unlock(&as->lock);
        interrupts_restore(ipl);

        return 0;
}


/** Handle page fault within the current address space.
 *
 * This is the high-level page fault handler. It decides whether the page fault
 * can be resolved by any backend and if so, it invokes the backend to resolve
 * the page fault.
 *
 * Interrupts are assumed disabled.
 *
 * @param page          Faulting page.
 * @param access        Access mode that caused the page fault (i.e.
 *                      read/write/exec).
 * @param istate        Pointer to the interrupted state.
 *
 * @return              AS_PF_FAULT on page fault, AS_PF_OK on success or
 *                      AS_PF_DEFER if the fault was caused by copy_to_uspace()
 *                      or copy_from_uspace().
 */
int as_page_fault(uintptr_t page, pf_access_t access, istate_t *istate)
{
        pte_t *pte;
        as_area_t *area;
        
        if (!THREAD)
                return AS_PF_FAULT;
                
        ASSERT(AS);

        mutex_lock(&AS->lock);
        area = find_area_and_lock(AS, page);    
        if (!area) {
                /*
                 * No area contained mapping for 'page'.
                 * Signal page fault to low-level handler.
                 */
                mutex_unlock(&AS->lock);
                goto page_fault;
        }

        if (area->attributes & AS_AREA_ATTR_PARTIAL) {
                /*
                 * The address space area is not fully initialized.
                 * Avoid possible race by returning error.
                 */
                mutex_unlock(&area->lock);
                mutex_unlock(&AS->lock);
                goto page_fault;                
        }

        if (!area->backend || !area->backend->page_fault) {
                /*
                 * The address space area is not backed by any backend
                 * or the backend cannot handle page faults.
                 */
                mutex_unlock(&area->lock);
                mutex_unlock(&AS->lock);
                goto page_fault;                
        }

        page_table_lock(AS, false);
        
        /*
         * To avoid race condition between two page faults on the same address,
         * we need to make sure the mapping has not been already inserted.
         */
        if ((pte = page_mapping_find(AS, page))) {
                if (PTE_PRESENT(pte)) {
                        if (((access == PF_ACCESS_READ) && PTE_READABLE(pte)) ||
                            (access == PF_ACCESS_WRITE && PTE_WRITABLE(pte)) ||
                            (access == PF_ACCESS_EXEC && PTE_EXECUTABLE(pte))) {
                                page_table_unlock(AS, false);
                                mutex_unlock(&area->lock);
                                mutex_unlock(&AS->lock);
                                return AS_PF_OK;
                        }
                }
        }
        
        /*
         * Resort to the backend page fault handler.
         */
        if (area->backend->page_fault(area, page, access) != AS_PF_OK) {
                page_table_unlock(AS, false);
                mutex_unlock(&area->lock);
                mutex_unlock(&AS->lock);
                goto page_fault;
        }
        
        page_table_unlock(AS, false);
        mutex_unlock(&area->lock);
        mutex_unlock(&AS->lock);
        return AS_PF_OK;

page_fault:
        if (THREAD->in_copy_from_uspace) {
                THREAD->in_copy_from_uspace = false;
                istate_set_retaddr(istate,
                    (uintptr_t) &memcpy_from_uspace_failover_address);
        } else if (THREAD->in_copy_to_uspace) {
                THREAD->in_copy_to_uspace = false;
                istate_set_retaddr(istate,
                    (uintptr_t) &memcpy_to_uspace_failover_address);
        } else {
                return AS_PF_FAULT;
        }

        return AS_PF_DEFER;
}

/** Switch address spaces.
 *
 * Note that this function cannot sleep as it is essentially a part of
 * scheduling. Sleeping here would lead to deadlock on wakeup. Another
 * thing which is forbidden in this context is locking the address space.
 *
 * When this function is enetered, no spinlocks may be held.
 *
 * @param old           Old address space or NULL.
 * @param new           New address space.
 */
void as_switch(as_t *old_as, as_t *new_as)
{
        DEADLOCK_PROBE_INIT(p_asidlock);
        preemption_disable();
retry:
        (void) interrupts_disable();
        if (!spinlock_trylock(&asidlock)) {
                /* 
                 * Avoid deadlock with TLB shootdown.
                 * We can enable interrupts here because
                 * preemption is disabled. We should not be
                 * holding any other lock.
                 */
                (void) interrupts_enable();
                DEADLOCK_PROBE(p_asidlock, DEADLOCK_THRESHOLD);
                goto retry;
        }
        preemption_enable();

        /*
         * First, take care of the old address space.
         */     
        if (old_as) {
                ASSERT(old_as->cpu_refcount);
                if((--old_as->cpu_refcount == 0) && (old_as != AS_KERNEL)) {
                        /*
                         * The old address space is no longer active on
                         * any processor. It can be appended to the
                         * list of inactive address spaces with assigned
                         * ASID.
                         */
                        ASSERT(old_as->asid != ASID_INVALID);
                        list_append(&old_as->inactive_as_with_asid_link,
                            &inactive_as_with_asid_head);
                }

                /*
                 * Perform architecture-specific tasks when the address space
                 * is being removed from the CPU.
                 */
                as_deinstall_arch(old_as);
        }

        /*
         * Second, prepare the new address space.
         */
        if ((new_as->cpu_refcount++ == 0) && (new_as != AS_KERNEL)) {
                if (new_as->asid != ASID_INVALID)
                        list_remove(&new_as->inactive_as_with_asid_link);
                else
                        new_as->asid = asid_get();
        }
#ifdef AS_PAGE_TABLE
        SET_PTL0_ADDRESS(new_as->genarch.page_table);
#endif
        
        /*
         * Perform architecture-specific steps.
         * (e.g. write ASID to hardware register etc.)
         */
        as_install_arch(new_as);

        spinlock_unlock(&asidlock);
        
        AS = new_as;
}

/** Convert address space area flags to page flags.
 *
 * @param aflags        Flags of some address space area.
 *
 * @return              Flags to be passed to page_mapping_insert().
 */
int area_flags_to_page_flags(int aflags)
{
        int flags;

        flags = PAGE_USER | PAGE_PRESENT;
        
        if (aflags & AS_AREA_READ)
                flags |= PAGE_READ;
                
        if (aflags & AS_AREA_WRITE)
                flags |= PAGE_WRITE;
        
        if (aflags & AS_AREA_EXEC)
                flags |= PAGE_EXEC;
        
        if (aflags & AS_AREA_CACHEABLE)
                flags |= PAGE_CACHEABLE;
                
        return flags;
}

/** Compute flags for virtual address translation subsytem.
 *
 * The address space area must be locked.
 * Interrupts must be disabled.
 *
 * @param a             Address space area.
 *
 * @return              Flags to be used in page_mapping_insert().
 */
int as_area_get_flags(as_area_t *a)
{
        return area_flags_to_page_flags(a->flags);
}

/** Create page table.
 *
 * Depending on architecture, create either address space private or global page
 * table.
 *
 * @param flags         Flags saying whether the page table is for the kernel
 *                      address space.
 *
 * @return              First entry of the page table.
 */
pte_t *page_table_create(int flags)
{
        ASSERT(as_operations);
        ASSERT(as_operations->page_table_create);
        
        return as_operations->page_table_create(flags);
}

/** Destroy page table.
 *
 * Destroy page table in architecture specific way.
 *
 * @param page_table    Physical address of PTL0.
 */
void page_table_destroy(pte_t *page_table)
{
        ASSERT(as_operations);
        ASSERT(as_operations->page_table_destroy);
        
        as_operations->page_table_destroy(page_table);
}

/** Lock page table.
 *
 * This function should be called before any page_mapping_insert(),
 * page_mapping_remove() and page_mapping_find().
 * 
 * Locking order is such that address space areas must be locked
 * prior to this call. Address space can be locked prior to this
 * call in which case the lock argument is false.
 *
 * @param as            Address space.
 * @param lock          If false, do not attempt to lock as->lock.
 */
void page_table_lock(as_t *as, bool lock)
{
        ASSERT(as_operations);
        ASSERT(as_operations->page_table_lock);
        
        as_operations->page_table_lock(as, lock);
}

/** Unlock page table.
 *
 * @param as            Address space.
 * @param unlock        If false, do not attempt to unlock as->lock.
 */
void page_table_unlock(as_t *as, bool unlock)
{
        ASSERT(as_operations);
        ASSERT(as_operations->page_table_unlock);
        
        as_operations->page_table_unlock(as, unlock);
}


/** Find address space area and lock it.
 *
 * The address space must be locked and interrupts must be disabled.
 *
 * @param as            Address space.
 * @param va            Virtual address.
 *
 * @return              Locked address space area containing va on success or
 *                      NULL on failure.
 */
as_area_t *find_area_and_lock(as_t *as, uintptr_t va)
{
        as_area_t *a;
        btree_node_t *leaf, *lnode;
        unsigned int i;
        
        a = (as_area_t *) btree_search(&as->as_area_btree, va, &leaf);
        if (a) {
                /* va is the base address of an address space area */
                mutex_lock(&a->lock);
                return a;
        }
        
        /*
         * Search the leaf node and the righmost record of its left neighbour
         * to find out whether this is a miss or va belongs to an address
         * space area found there.
         */
        
        /* First, search the leaf node itself. */
        for (i = 0; i < leaf->keys; i++) {
                a = (as_area_t *) leaf->value[i];
                mutex_lock(&a->lock);
                if ((a->base <= va) && (va < a->base + a->pages * PAGE_SIZE)) {
                        return a;
                }
                mutex_unlock(&a->lock);
        }

        /*
         * Second, locate the left neighbour and test its last record.
         * Because of its position in the B+tree, it must have base < va.
         */
        lnode = btree_leaf_node_left_neighbour(&as->as_area_btree, leaf);
        if (lnode) {
                a = (as_area_t *) lnode->value[lnode->keys - 1];
                mutex_lock(&a->lock);
                if (va < a->base + a->pages * PAGE_SIZE) {
                        return a;
                }
                mutex_unlock(&a->lock);
        }

        return NULL;
}

/** Check area conflicts with other areas.
 *
 * The address space must be locked and interrupts must be disabled.
 *
 * @param as            Address space.
 * @param va            Starting virtual address of the area being tested.
 * @param size          Size of the area being tested.
 * @param avoid_area    Do not touch this area. 
 *
 * @return              True if there is no conflict, false otherwise.
 */
bool
check_area_conflicts(as_t *as, uintptr_t va, size_t size, as_area_t *avoid_area)
{
        as_area_t *a;
        btree_node_t *leaf, *node;
        unsigned int i;
        
        /*
         * We don't want any area to have conflicts with NULL page.
         */
        if (overlaps(va, size, NULL, PAGE_SIZE))
                return false;
        
        /*
         * The leaf node is found in O(log n), where n is proportional to
         * the number of address space areas belonging to as.
         * The check for conflicts is then attempted on the rightmost
         * record in the left neighbour, the leftmost record in the right
         * neighbour and all records in the leaf node itself.
         */
        
        if ((a = (as_area_t *) btree_search(&as->as_area_btree, va, &leaf))) {
                if (a != avoid_area)
                        return false;
        }
        
        /* First, check the two border cases. */
        if ((node = btree_leaf_node_left_neighbour(&as->as_area_btree, leaf))) {
                a = (as_area_t *) node->value[node->keys - 1];
                mutex_lock(&a->lock);
                if (overlaps(va, size, a->base, a->pages * PAGE_SIZE)) {
                        mutex_unlock(&a->lock);
                        return false;
                }
                mutex_unlock(&a->lock);
        }
        node = btree_leaf_node_right_neighbour(&as->as_area_btree, leaf);
        if (node) {
                a = (as_area_t *) node->value[0];
                mutex_lock(&a->lock);
                if (overlaps(va, size, a->base, a->pages * PAGE_SIZE)) {
                        mutex_unlock(&a->lock);
                        return false;
                }
                mutex_unlock(&a->lock);
        }
        
        /* Second, check the leaf node. */
        for (i = 0; i < leaf->keys; i++) {
                a = (as_area_t *) leaf->value[i];
        
                if (a == avoid_area)
                        continue;
        
                mutex_lock(&a->lock);
                if (overlaps(va, size, a->base, a->pages * PAGE_SIZE)) {
                        mutex_unlock(&a->lock);
                        return false;
                }
                mutex_unlock(&a->lock);
        }

        /*
         * So far, the area does not conflict with other areas.
         * Check if it doesn't conflict with kernel address space.
         */      
        if (!KERNEL_ADDRESS_SPACE_SHADOWED) {
                return !overlaps(va, size, 
                    KERNEL_ADDRESS_SPACE_START,
                    KERNEL_ADDRESS_SPACE_END - KERNEL_ADDRESS_SPACE_START);
        }

        return true;
}

/** Return size of the address space area with given base.
 *
 * @param base          Arbitrary address insede the address space area.
 *
 * @return              Size of the address space area in bytes or zero if it
 *                      does not exist.
 */
size_t as_area_get_size(uintptr_t base)
{
        ipl_t ipl;
        as_area_t *src_area;
        size_t size;

        ipl = interrupts_disable();
        src_area = find_area_and_lock(AS, base);
        if (src_area) {
                size = src_area->pages * PAGE_SIZE;
                mutex_unlock(&src_area->lock);
        } else {
                size = 0;
        }
        interrupts_restore(ipl);
        return size;
}

/** Mark portion of address space area as used.
 *
 * The address space area must be already locked.
 *
 * @param a             Address space area.
 * @param page          First page to be marked.
 * @param count         Number of page to be marked.
 *
 * @return              Zero on failure and non-zero on success.
 */
int used_space_insert(as_area_t *a, uintptr_t page, count_t count)
{
        btree_node_t *leaf, *node;
        count_t pages;
        unsigned int i;

        ASSERT(page == ALIGN_DOWN(page, PAGE_SIZE));
        ASSERT(count);

        pages = (count_t) btree_search(&a->used_space, page, &leaf);
        if (pages) {
                /*
                 * We hit the beginning of some used space.
                 */
                return 0;
        }

        if (!leaf->keys) {
                btree_insert(&a->used_space, page, (void *) count, leaf);
                return 1;
        }

        node = btree_leaf_node_left_neighbour(&a->used_space, leaf);
        if (node) {
                uintptr_t left_pg = node->key[node->keys - 1];
                uintptr_t right_pg = leaf->key[0];
                count_t left_cnt = (count_t) node->value[node->keys - 1];
                count_t right_cnt = (count_t) leaf->value[0];
                
                /*
                 * Examine the possibility that the interval fits
                 * somewhere between the rightmost interval of
                 * the left neigbour and the first interval of the leaf.
                 */
                 
                if (page >= right_pg) {
                        /* Do nothing. */
                } else if (overlaps(page, count * PAGE_SIZE, left_pg,
                    left_cnt * PAGE_SIZE)) {
                        /* The interval intersects with the left interval. */
                        return 0;
                } else if (overlaps(page, count * PAGE_SIZE, right_pg,
                    right_cnt * PAGE_SIZE)) {
                        /* The interval intersects with the right interval. */
                        return 0;                       
                } else if ((page == left_pg + left_cnt * PAGE_SIZE) &&
                    (page + count * PAGE_SIZE == right_pg)) {
                        /*
                         * The interval can be added by merging the two already
                         * present intervals.
                         */
                        node->value[node->keys - 1] += count + right_cnt;
                        btree_remove(&a->used_space, right_pg, leaf);
                        return 1; 
                } else if (page == left_pg + left_cnt * PAGE_SIZE) {
                        /* 
                         * The interval can be added by simply growing the left
                         * interval.
                         */
                        node->value[node->keys - 1] += count;
                        return 1;
                } else if (page + count * PAGE_SIZE == right_pg) {
                        /*
                         * The interval can be addded by simply moving base of
                         * the right interval down and increasing its size
                         * accordingly.
                         */
                        leaf->value[0] += count;
                        leaf->key[0] = page;
                        return 1;
                } else {
                        /*
                         * The interval is between both neigbouring intervals,
                         * but cannot be merged with any of them.
                         */
                        btree_insert(&a->used_space, page, (void *) count,
                            leaf);
                        return 1;
                }
        } else if (page < leaf->key[0]) {
                uintptr_t right_pg = leaf->key[0];
                count_t right_cnt = (count_t) leaf->value[0];
        
                /*
                 * Investigate the border case in which the left neighbour does
                 * not exist but the interval fits from the left.
                 */
                 
                if (overlaps(page, count * PAGE_SIZE, right_pg,
                    right_cnt * PAGE_SIZE)) {
                        /* The interval intersects with the right interval. */
                        return 0;
                } else if (page + count * PAGE_SIZE == right_pg) {
                        /*
                         * The interval can be added by moving the base of the
                         * right interval down and increasing its size
                         * accordingly.
                         */
                        leaf->key[0] = page;
                        leaf->value[0] += count;
                        return 1;
                } else {
                        /*
                         * The interval doesn't adjoin with the right interval.
                         * It must be added individually.
                         */
                        btree_insert(&a->used_space, page, (void *) count,
                            leaf);
                        return 1;
                }
        }

        node = btree_leaf_node_right_neighbour(&a->used_space, leaf);
        if (node) {
                uintptr_t left_pg = leaf->key[leaf->keys - 1];
                uintptr_t right_pg = node->key[0];
                count_t left_cnt = (count_t) leaf->value[leaf->keys - 1];
                count_t right_cnt = (count_t) node->value[0];
                
                /*
                 * Examine the possibility that the interval fits
                 * somewhere between the leftmost interval of
                 * the right neigbour and the last interval of the leaf.
                 */

                if (page < left_pg) {
                        /* Do nothing. */
                } else if (overlaps(page, count * PAGE_SIZE, left_pg,
                    left_cnt * PAGE_SIZE)) {
                        /* The interval intersects with the left interval. */
                        return 0;
                } else if (overlaps(page, count * PAGE_SIZE, right_pg,
                    right_cnt * PAGE_SIZE)) {
                        /* The interval intersects with the right interval. */
                        return 0;                       
                } else if ((page == left_pg + left_cnt * PAGE_SIZE) &&
                    (page + count * PAGE_SIZE == right_pg)) {
                        /*
                         * The interval can be added by merging the two already
                         * present intervals.
                         * */
                        leaf->value[leaf->keys - 1] += count + right_cnt;
                        btree_remove(&a->used_space, right_pg, node);
                        return 1; 
                } else if (page == left_pg + left_cnt * PAGE_SIZE) {
                        /*
                         * The interval can be added by simply growing the left
                         * interval.
                         * */
                        leaf->value[leaf->keys - 1] +=  count;
                        return 1;
                } else if (page + count * PAGE_SIZE == right_pg) {
                        /*
                         * The interval can be addded by simply moving base of
                         * the right interval down and increasing its size
                         * accordingly.
                         */
                        node->value[0] += count;
                        node->key[0] = page;
                        return 1;
                } else {
                        /*
                         * The interval is between both neigbouring intervals,
                         * but cannot be merged with any of them.
                         */
                        btree_insert(&a->used_space, page, (void *) count,
                            leaf);
                        return 1;
                }
        } else if (page >= leaf->key[leaf->keys - 1]) {
                uintptr_t left_pg = leaf->key[leaf->keys - 1];
                count_t left_cnt = (count_t) leaf->value[leaf->keys - 1];
        
                /*
                 * Investigate the border case in which the right neighbour
                 * does not exist but the interval fits from the right.
                 */
                 
                if (overlaps(page, count * PAGE_SIZE, left_pg,
                    left_cnt * PAGE_SIZE)) {
                        /* The interval intersects with the left interval. */
                        return 0;
                } else if (left_pg + left_cnt * PAGE_SIZE == page) {
                        /*
                         * The interval can be added by growing the left
                         * interval.
                         */
                        leaf->value[leaf->keys - 1] += count;
                        return 1;
                } else {
                        /*
                         * The interval doesn't adjoin with the left interval.
                         * It must be added individually.
                         */
                        btree_insert(&a->used_space, page, (void *) count,
                            leaf);
                        return 1;
                }
        }
        
        /*
         * Note that if the algorithm made it thus far, the interval can fit
         * only between two other intervals of the leaf. The two border cases
         * were already resolved.
         */
        for (i = 1; i < leaf->keys; i++) {
                if (page < leaf->key[i]) {
                        uintptr_t left_pg = leaf->key[i - 1];
                        uintptr_t right_pg = leaf->key[i];
                        count_t left_cnt = (count_t) leaf->value[i - 1];
                        count_t right_cnt = (count_t) leaf->value[i];

                        /*
                         * The interval fits between left_pg and right_pg.
                         */

                        if (overlaps(page, count * PAGE_SIZE, left_pg,
                            left_cnt * PAGE_SIZE)) {
                                /*
                                 * The interval intersects with the left
                                 * interval.
                                 */
                                return 0;
                        } else if (overlaps(page, count * PAGE_SIZE, right_pg,
                            right_cnt * PAGE_SIZE)) {
                                /*
                                 * The interval intersects with the right
                                 * interval.
                                 */
                                return 0;                       
                        } else if ((page == left_pg + left_cnt * PAGE_SIZE) &&
                            (page + count * PAGE_SIZE == right_pg)) {
                                /*
                                 * The interval can be added by merging the two
                                 * already present intervals.
                                 */
                                leaf->value[i - 1] += count + right_cnt;
                                btree_remove(&a->used_space, right_pg, leaf);
                                return 1; 
                        } else if (page == left_pg + left_cnt * PAGE_SIZE) {
                                /*
                                 * The interval can be added by simply growing
                                 * the left interval.
                                 */
                                leaf->value[i - 1] += count;
                                return 1;
                        } else if (page + count * PAGE_SIZE == right_pg) {
                                /*
                                 * The interval can be addded by simply moving
                                 * base of the right interval down and
                                 * increasing its size accordingly.
                                 */
                                leaf->value[i] += count;
                                leaf->key[i] = page;
                                return 1;
                        } else {
                                /*
                                 * The interval is between both neigbouring
                                 * intervals, but cannot be merged with any of
                                 * them.
                                 */
                                btree_insert(&a->used_space, page,
                                    (void *) count, leaf);
                                return 1;
                        }
                }
        }

        panic("Inconsistency detected while adding %" PRIc " pages of used "
            "space at %p.\n", count, page);
}

/** Mark portion of address space area as unused.
 *
 * The address space area must be already locked.
 *
 * @param a             Address space area.
 * @param page          First page to be marked.
 * @param count         Number of page to be marked.
 *
 * @return              Zero on failure and non-zero on success.
 */
int used_space_remove(as_area_t *a, uintptr_t page, count_t count)
{
        btree_node_t *leaf, *node;
        count_t pages;
        unsigned int i;

        ASSERT(page == ALIGN_DOWN(page, PAGE_SIZE));
        ASSERT(count);

        pages = (count_t) btree_search(&a->used_space, page, &leaf);
        if (pages) {
                /*
                 * We are lucky, page is the beginning of some interval.
                 */
                if (count > pages) {
                        return 0;
                } else if (count == pages) {
                        btree_remove(&a->used_space, page, leaf);
                        return 1;
                } else {
                        /*
                         * Find the respective interval.
                         * Decrease its size and relocate its start address.
                         */
                        for (i = 0; i < leaf->keys; i++) {
                                if (leaf->key[i] == page) {
                                        leaf->key[i] += count * PAGE_SIZE;
                                        leaf->value[i] -= count;
                                        return 1;
                                }
                        }
                        goto error;
                }
        }

        node = btree_leaf_node_left_neighbour(&a->used_space, leaf);
        if (node && page < leaf->key[0]) {
                uintptr_t left_pg = node->key[node->keys - 1];
                count_t left_cnt = (count_t) node->value[node->keys - 1];

                if (overlaps(left_pg, left_cnt * PAGE_SIZE, page,
                    count * PAGE_SIZE)) {
                        if (page + count * PAGE_SIZE ==
                            left_pg + left_cnt * PAGE_SIZE) {
                                /*
                                 * The interval is contained in the rightmost
                                 * interval of the left neighbour and can be
                                 * removed by updating the size of the bigger
                                 * interval.
                                 */
                                node->value[node->keys - 1] -= count;
                                return 1;
                        } else if (page + count * PAGE_SIZE <
                            left_pg + left_cnt*PAGE_SIZE) {
                                count_t new_cnt;
                                
                                /*
                                 * The interval is contained in the rightmost
                                 * interval of the left neighbour but its
                                 * removal requires both updating the size of
                                 * the original interval and also inserting a
                                 * new interval.
                                 */
                                new_cnt = ((left_pg + left_cnt * PAGE_SIZE) -
                                    (page + count*PAGE_SIZE)) >> PAGE_WIDTH;
                                node->value[node->keys - 1] -= count + new_cnt;
                                btree_insert(&a->used_space, page +
                                    count * PAGE_SIZE, (void *) new_cnt, leaf);
                                return 1;
                        }
                }
                return 0;
        } else if (page < leaf->key[0]) {
                return 0;
        }
        
        if (page > leaf->key[leaf->keys - 1]) {
                uintptr_t left_pg = leaf->key[leaf->keys - 1];
                count_t left_cnt = (count_t) leaf->value[leaf->keys - 1];

                if (overlaps(left_pg, left_cnt * PAGE_SIZE, page,
                    count * PAGE_SIZE)) {
                        if (page + count * PAGE_SIZE == 
                            left_pg + left_cnt * PAGE_SIZE) {
                                /*
                                 * The interval is contained in the rightmost
                                 * interval of the leaf and can be removed by
                                 * updating the size of the bigger interval.
                                 */
                                leaf->value[leaf->keys - 1] -= count;
                                return 1;
                        } else if (page + count * PAGE_SIZE < left_pg +
                            left_cnt * PAGE_SIZE) {
                                count_t new_cnt;
                                
                                /*
                                 * The interval is contained in the rightmost
                                 * interval of the leaf but its removal
                                 * requires both updating the size of the
                                 * original interval and also inserting a new
                                 * interval.
                                 */
                                new_cnt = ((left_pg + left_cnt * PAGE_SIZE) -
                                    (page + count * PAGE_SIZE)) >> PAGE_WIDTH;
                                leaf->value[leaf->keys - 1] -= count + new_cnt;
                                btree_insert(&a->used_space, page +
                                    count * PAGE_SIZE, (void *) new_cnt, leaf);
                                return 1;
                        }
                }
                return 0;
        }       
        
        /*
         * The border cases have been already resolved.
         * Now the interval can be only between intervals of the leaf. 
         */
        for (i = 1; i < leaf->keys - 1; i++) {
                if (page < leaf->key[i]) {
                        uintptr_t left_pg = leaf->key[i - 1];
                        count_t left_cnt = (count_t) leaf->value[i - 1];

                        /*
                         * Now the interval is between intervals corresponding
                         * to (i - 1) and i.
                         */
                        if (overlaps(left_pg, left_cnt * PAGE_SIZE, page,
                            count * PAGE_SIZE)) {
                                if (page + count * PAGE_SIZE ==
                                    left_pg + left_cnt*PAGE_SIZE) {
                                        /*
                                         * The interval is contained in the
                                         * interval (i - 1) of the leaf and can
                                         * be removed by updating the size of
                                         * the bigger interval.
                                         */
                                        leaf->value[i - 1] -= count;
                                        return 1;
                                } else if (page + count * PAGE_SIZE <
                                    left_pg + left_cnt * PAGE_SIZE) {
                                        count_t new_cnt;
                                
                                        /*
                                         * The interval is contained in the
                                         * interval (i - 1) of the leaf but its
                                         * removal requires both updating the
                                         * size of the original interval and
                                         * also inserting a new interval.
                                         */
                                        new_cnt = ((left_pg +
                                            left_cnt * PAGE_SIZE) -
                                            (page + count * PAGE_SIZE)) >>
                                            PAGE_WIDTH;
                                        leaf->value[i - 1] -= count + new_cnt;
                                        btree_insert(&a->used_space, page +
                                            count * PAGE_SIZE, (void *) new_cnt,
                                            leaf);
                                        return 1;
                                }
                        }
                        return 0;
                }
        }

error:
        panic("Inconsistency detected while removing %" PRIc " pages of used "
            "space from %p.\n", count, page);
}

/** Remove reference to address space area share info.
 *
 * If the reference count drops to 0, the sh_info is deallocated.
 *
 * @param sh_info       Pointer to address space area share info.
 */
void sh_info_remove_reference(share_info_t *sh_info)
{
        bool dealloc = false;

        mutex_lock(&sh_info->lock);
        ASSERT(sh_info->refcount);
        if (--sh_info->refcount == 0) {
                dealloc = true;
                link_t *cur;
                
                /*
                 * Now walk carefully the pagemap B+tree and free/remove
                 * reference from all frames found there.
                 */
                for (cur = sh_info->pagemap.leaf_head.next;
                    cur != &sh_info->pagemap.leaf_head; cur = cur->next) {
                        btree_node_t *node;
                        unsigned int i;
                        
                        node = list_get_instance(cur, btree_node_t, leaf_link);
                        for (i = 0; i < node->keys; i++) 
                                frame_free((uintptr_t) node->value[i]);
                }
                
        }
        mutex_unlock(&sh_info->lock);
        
        if (dealloc) {
                btree_destroy(&sh_info->pagemap);
                free(sh_info);
        }
}

/*
 * Address space related syscalls.
 */

/** Wrapper for as_area_create(). */
unative_t sys_as_area_create(uintptr_t address, size_t size, int flags)
{
        if (as_area_create(AS, flags | AS_AREA_CACHEABLE, size, address,
            AS_AREA_ATTR_NONE, &anon_backend, NULL))
                return (unative_t) address;
        else
                return (unative_t) -1;
}

/** Wrapper for as_area_resize(). */
unative_t sys_as_area_resize(uintptr_t address, size_t size, int flags)
{
        return (unative_t) as_area_resize(AS, address, size, 0);
}

/** Wrapper for as_area_change_flags(). */
unative_t sys_as_area_change_flags(uintptr_t address, int flags)
{
        return (unative_t) as_area_change_flags(AS, flags, address);
}

/** Wrapper for as_area_destroy(). */
unative_t sys_as_area_destroy(uintptr_t address)
{
        return (unative_t) as_area_destroy(AS, address);
}

/** Print out information about address space.
 *
 * @param as            Address space.
 */
void as_print(as_t *as)
{
        ipl_t ipl;
        
        ipl = interrupts_disable();
        mutex_lock(&as->lock);
        
        /* print out info about address space areas */
        link_t *cur;
        for (cur = as->as_area_btree.leaf_head.next;
            cur != &as->as_area_btree.leaf_head; cur = cur->next) {
                btree_node_t *node;
                
                node = list_get_instance(cur, btree_node_t, leaf_link);
                
                unsigned int i;
                for (i = 0; i < node->keys; i++) {
                        as_area_t *area = node->value[i];
                
                        mutex_lock(&area->lock);
                        printf("as_area: %p, base=%p, pages=%" PRIc
                            " (%p - %p)\n", area, area->base, area->pages,
                            area->base, area->base + FRAMES2SIZE(area->pages));
                        mutex_unlock(&area->lock);
                }
        }
        
        mutex_unlock(&as->lock);
        interrupts_restore(ipl);
}

/** @}
 */