source: mainline/generic/src/mm/as.c@ 7242a78e

/*
 * 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.
 */

/**
 * @file        as.c
 * @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 space 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 <synch/spinlock.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 <arch/types.h>
#include <typedefs.h>
#include <syscall/copy.h>
#include <arch/interrupt.h>

as_operations_t *as_operations = NULL;

/** Address space lock. It protects inactive_as_with_asid_head. */
SPINLOCK_INITIALIZE(as_lock);

/**
 * 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 aflags);
static int get_area_flags(as_area_t *a);
static as_area_t *find_area_and_lock(as_t *as, __address va);
static bool check_area_conflicts(as_t *as, __address va, size_t size, as_area_t *avoid_area);

/** Initialize address space subsystem. */
void as_init(void)
{
        as_arch_init();
        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 way in wich the address space is created.
 */
as_t *as_create(int flags)
{
        as_t *as;

        as = (as_t *) malloc(sizeof(as_t), 0);
        link_initialize(&as->inactive_as_with_asid_link);
        spinlock_initialize(&as->lock, "as_lock");
        btree_create(&as->as_area_btree);
        
        if (flags & FLAG_AS_KERNEL)
                as->asid = ASID_KERNEL;
        else
                as->asid = ASID_INVALID;
        
        as->refcount = 0;
        as->page_table = page_table_create(flags);

        return as;
}

/** Free Adress space */
void as_free(as_t *as)
{
        ASSERT(as->refcount == 0);

        /* TODO: free as_areas and other resources held by as */
        /* TODO: free page table */
        free(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.
 *
 * @return Address space area on success or NULL on failure.
 */
as_area_t *as_area_create(as_t *as, int flags, size_t size, __address base, int attrs)
{
        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();
        spinlock_lock(&as->lock);
        
        if (!check_area_conflicts(as, base, size, NULL)) {
                spinlock_unlock(&as->lock);
                interrupts_restore(ipl);
                return NULL;
        }
        
        a = (as_area_t *) malloc(sizeof(as_area_t), 0);

        spinlock_initialize(&a->lock, "as_area_lock");
        
        a->flags = flags;
        a->attributes = attrs;
        a->pages = SIZE2FRAMES(size);
        a->base = base;
        
        btree_insert(&as->as_area_btree, base, (void *) a, NULL);

        spinlock_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, __address address, size_t size, int flags)
{
        as_area_t *area;
        ipl_t ipl;
        size_t pages;
        
        ipl = interrupts_disable();
        spinlock_lock(&as->lock);
        
        /*
         * Locate the area.
         */
        area = find_area_and_lock(as, address);
        if (!area) {
                spinlock_unlock(&as->lock);
                interrupts_restore(ipl);
                return ENOENT;
        }

        if (area->flags & AS_AREA_DEVICE) {
                /*
                 * Remapping of address space areas associated
                 * with memory mapped devices is not supported.
                 */
                spinlock_unlock(&area->lock);
                spinlock_unlock(&as->lock);
                interrupts_restore(ipl);
                return ENOTSUP;
        }

        pages = SIZE2FRAMES((address - area->base) + size);
        if (!pages) {
                /*
                 * Zero size address space areas are not allowed.
                 */
                spinlock_unlock(&area->lock);
                spinlock_unlock(&as->lock);
                interrupts_restore(ipl);
                return EPERM;
        }
        
        if (pages < area->pages) {
                int i;

                /*
                 * Shrinking the area.
                 * No need to check for overlaps.
                 */
                for (i = pages; i < area->pages; i++) {
                        pte_t *pte;
                        
                        /*
                         * Releasing physical memory.
                         * This depends on the fact that the memory was allocated using frame_alloc().
                         */
                        page_table_lock(as, false);
                        pte = page_mapping_find(as, area->base + i*PAGE_SIZE);
                        if (pte && PTE_VALID(pte)) {
                                __address frame;

                                ASSERT(PTE_PRESENT(pte));
                                frame = PTE_GET_FRAME(pte);
                                page_mapping_remove(as, area->base + i*PAGE_SIZE);
                                page_table_unlock(as, false);

                                frame_free(ADDR2PFN(frame));
                        } else {
                                page_table_unlock(as, false);
                        }
                }
                /*
                 * Invalidate TLB's.
                 */
                tlb_shootdown_start(TLB_INVL_PAGES, AS->asid, area->base + pages*PAGE_SIZE, area->pages - pages);
                tlb_invalidate_pages(AS->asid, 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)) {
                        spinlock_unlock(&area->lock);
                        spinlock_unlock(&as->lock);             
                        interrupts_restore(ipl);
                        return EADDRNOTAVAIL;
                }
        } 

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

        return 0;
}

/** Destroy address space area.
 *
 * @param as Address space.
 * @param address Address withing 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, __address address)
{
        as_area_t *area;
        __address base;
        ipl_t ipl;
        int i;

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

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

        base = area->base;      
        for (i = 0; i < area->pages; i++) {
                pte_t *pte;

                /*
                 * Releasing physical memory.
                 * Areas mapping memory-mapped devices are treated differently than
                 * areas backing frame_alloc()'ed memory.
                 */
                page_table_lock(as, false);
                pte = page_mapping_find(as, area->base + i*PAGE_SIZE);
                if (pte && PTE_VALID(pte)) {
                        ASSERT(PTE_PRESENT(pte));
                        page_mapping_remove(as, area->base + i*PAGE_SIZE);
                        if (area->flags & AS_AREA_DEVICE) {
                                __address frame;
                                frame = PTE_GET_FRAME(pte);
                                frame_free(ADDR2PFN(frame));
                        }
                        page_table_unlock(as, false);
                } else {
                        page_table_unlock(as, false);
                }
        }
        /*
         * Invalidate TLB's.
         */
        tlb_shootdown_start(TLB_INVL_PAGES, AS->asid, area->base, area->pages);
        tlb_invalidate_pages(AS->asid, area->base, area->pages);
        tlb_shootdown_finalize();

        spinlock_unlock(&area->lock);

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

/** Send address space area to another task.
 *
 * Address space area is sent to the specified task.
 * If the destination task is willing to accept the
 * area, a new area is created according to the
 * source area. Moreover, any existing mapping
 * is copied as well, providing thus a mechanism
 * for sharing group of pages. The source address
 * space area and any associated mapping is preserved.
 *
 * @param dst_id Task ID of the accepting task.
 * @param src_base Base address of the source address space area.
 *
 * @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.
 */
int as_area_send(task_id_t dst_id, __address src_base)
{
        ipl_t ipl;
        task_t *t;
        count_t i;
        as_t *dst_as;
        __address dst_base;
        int src_flags;
        size_t src_size;
        as_area_t *src_area, *dst_area;
        
        ipl = interrupts_disable();
        spinlock_lock(&tasks_lock);
        
        t = task_find_by_id(dst_id);
        if (!NULL) {
                spinlock_unlock(&tasks_lock);
                interrupts_restore(ipl);
                return ENOENT;
        }

        spinlock_lock(&t->lock);
        spinlock_unlock(&tasks_lock);

        dst_as = t->as;
        dst_base = (__address) t->accept_arg.base;
        
        if (dst_as == AS) {
                /*
                 * The two tasks share the entire address space.
                 * Return error since there is no point in continuing.
                 */
                spinlock_unlock(&t->lock);
                interrupts_restore(ipl);
                return EPERM;
        }
        
        spinlock_lock(&AS->lock);
        src_area = find_area_and_lock(AS, src_base);
        if (!src_area) {
                /*
                 * Could not find the source address space area.
                 */
                spinlock_unlock(&t->lock);
                spinlock_unlock(&AS->lock);
                interrupts_restore(ipl);
                return ENOENT;
        }
        src_size = src_area->pages * PAGE_SIZE;
        src_flags = src_area->flags;
        spinlock_unlock(&src_area->lock);
        spinlock_unlock(&AS->lock);

        if ((t->accept_arg.task_id != TASK->taskid) || (t->accept_arg.size != src_size) ||
            (t->accept_arg.flags != src_flags)) {
                /*
                 * Discrepancy in either task ID, size or flags.
                 */
                spinlock_unlock(&t->lock);
                interrupts_restore(ipl);
                return EPERM;
        }
        
        /*
         * 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.
         */
        dst_area = as_area_create(dst_as, src_flags, src_size, dst_base, AS_AREA_ATTR_PARTIAL);
        if (!dst_area) {
                /*
                 * Destination address space area could not be created.
                 */
                spinlock_unlock(&t->lock);
                interrupts_restore(ipl);
                return ENOMEM;
        }
        
        memsetb((__address) &t->accept_arg, sizeof(as_area_acptsnd_arg_t), 0);
        spinlock_unlock(&t->lock);
        
        /*
         * Avoid deadlock by first locking the address space with lower address.
         */
        if (dst_as < AS) {
                spinlock_lock(&dst_as->lock);
                spinlock_lock(&AS->lock);
        } else {
                spinlock_lock(&AS->lock);
                spinlock_lock(&dst_as->lock);
        }
        
        for (i = 0; i < SIZE2FRAMES(src_size); i++) {
                pte_t *pte;
                __address frame;
                        
                page_table_lock(AS, false);
                pte = page_mapping_find(AS, src_base + i*PAGE_SIZE);
                if (pte && PTE_VALID(pte)) {
                        ASSERT(PTE_PRESENT(pte));
                        frame = PTE_GET_FRAME(pte);
                        if (!(src_flags & AS_AREA_DEVICE))
                                frame_reference_add(ADDR2PFN(frame));
                        page_table_unlock(AS, false);
                } else {
                        page_table_unlock(AS, false);
                        continue;
                }
                
                page_table_lock(dst_as, false);
                page_mapping_insert(dst_as, dst_base + i*PAGE_SIZE, frame, area_flags_to_page_flags(src_flags));
                page_table_unlock(dst_as, false);
        }

        /*
         * Now the destination address space area has been
         * fully initialized. Clear the AS_AREA_ATTR_PARTIAL
         * attribute.
         */     
        spinlock_lock(&dst_area->lock);
        dst_area->attributes &= ~AS_AREA_ATTR_PARTIAL;
        spinlock_unlock(&dst_area->lock);
        
        spinlock_unlock(&AS->lock);
        spinlock_unlock(&dst_as->lock);
        interrupts_restore(ipl);
        
        return 0;
}

/** Initialize mapping for one page of address space.
 *
 * This functions maps 'page' to 'frame' according
 * to attributes of the address space area to
 * wich 'page' belongs.
 *
 * @param as Target address space.
 * @param page Virtual page within the area.
 * @param frame Physical frame to which page will be mapped.
 */
void as_set_mapping(as_t *as, __address page, __address frame)
{
        as_area_t *area;
        ipl_t ipl;
        
        ipl = interrupts_disable();
        page_table_lock(as, true);
        
        area = find_area_and_lock(as, page);
        if (!area) {
                panic("page not part of any as_area\n");
        }

        page_mapping_insert(as, page, frame, get_area_flags(area));
        
        spinlock_unlock(&area->lock);
        page_table_unlock(as, true);
        interrupts_restore(ipl);
}

/** Handle page fault within the current address space.
 *
 * This is the high-level page fault handler.
 * Interrupts are assumed disabled.
 *
 * @param page Faulting page.
 * @param istate Pointer to interrupted state.
 *
 * @return 0 on page fault, 1 on success or 2 if the fault was caused by copy_to_uspace() or copy_from_uspace().
 */
int as_page_fault(__address page, istate_t *istate)
{
        pte_t *pte;
        as_area_t *area;
        __address frame;
        
        ASSERT(AS);

        spinlock_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.
                 */
                spinlock_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.
                 */
                spinlock_unlock(&area->lock);
                spinlock_unlock(&AS->lock);
                goto page_fault;                
        }

        ASSERT(!(area->flags & AS_AREA_DEVICE));

        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)) {
                        page_table_unlock(AS, false);
                        spinlock_unlock(&area->lock);
                        spinlock_unlock(&AS->lock);
                        return 1;
                }
        }

        /*
         * In general, there can be several reasons that
         * can have caused this fault.
         *
         * - non-existent mapping: the area is a scratch
         *   area (e.g. stack) and so far has not been
         *   allocated a frame for the faulting page
         *
         * - non-present mapping: another possibility,
         *   currently not implemented, would be frame
         *   reuse; when this becomes a possibility,
         *   do not forget to distinguish between
         *   the different causes
         */
        frame = PFN2ADDR(frame_alloc(ONE_FRAME, 0));
        memsetb(PA2KA(frame), FRAME_SIZE, 0);
        
        /*
         * Map 'page' to 'frame'.
         * Note that TLB shootdown is not attempted as only new information is being
         * inserted into page tables.
         */
        page_mapping_insert(AS, page, frame, get_area_flags(area));
        page_table_unlock(AS, false);
        
        spinlock_unlock(&area->lock);
        spinlock_unlock(&AS->lock);
        return AS_PF_OK;

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

        return AS_PF_DEFER;
}

/** Switch address spaces.
 *
 * @param old Old address space or NULL.
 * @param new New address space.
 */
void as_switch(as_t *old, as_t *new)
{
        ipl_t ipl;
        bool needs_asid = false;
        
        ipl = interrupts_disable();
        spinlock_lock(&as_lock);

        /*
         * First, take care of the old address space.
         */     
        if (old) {
                spinlock_lock(&old->lock);
                ASSERT(old->refcount);
                if((--old->refcount == 0) && (old != 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->asid != ASID_INVALID);
                         list_append(&old->inactive_as_with_asid_link, &inactive_as_with_asid_head);
                }
                spinlock_unlock(&old->lock);
        }

        /*
         * Second, prepare the new address space.
         */
        spinlock_lock(&new->lock);
        if ((new->refcount++ == 0) && (new != AS_KERNEL)) {
                if (new->asid != ASID_INVALID)
                        list_remove(&new->inactive_as_with_asid_link);
                else
                        needs_asid = true;      /* defer call to asid_get() until new->lock is released */
        }
        SET_PTL0_ADDRESS(new->page_table);
        spinlock_unlock(&new->lock);

        if (needs_asid) {
                /*
                 * Allocation of new ASID was deferred
                 * until now in order to avoid deadlock.
                 */
                asid_t asid;
                
                asid = asid_get();
                spinlock_lock(&new->lock);
                new->asid = asid;
                spinlock_unlock(&new->lock);
        }
        spinlock_unlock(&as_lock);
        interrupts_restore(ipl);
        
        /*
         * Perform architecture-specific steps.
         * (e.g. write ASID to hardware register etc.)
         */
        as_install_arch(new);
        
        AS = new;
}

/** 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_DEVICE))
                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 get_area_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 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);
}

/** 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, __address va)
{
        as_area_t *a;
        btree_node_t *leaf, *lnode;
        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 */
                spinlock_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];
                spinlock_lock(&a->lock);
                if ((a->base <= va) && (va < a->base + a->pages * PAGE_SIZE)) {
                        return a;
                }
                spinlock_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.
         */
        if ((lnode = btree_leaf_node_left_neighbour(&as->as_area_btree, leaf))) {
                a = (as_area_t *) lnode->value[lnode->keys - 1];
                spinlock_lock(&a->lock);
                if (va < a->base + a->pages * PAGE_SIZE) {
                        return a;
                }
                spinlock_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, __address va, size_t size, as_area_t *avoid_area)
{
        as_area_t *a;
        btree_node_t *leaf, *node;
        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];
                spinlock_lock(&a->lock);
                if (overlaps(va, size, a->base, a->pages * PAGE_SIZE)) {
                        spinlock_unlock(&a->lock);
                        return false;
                }
                spinlock_unlock(&a->lock);
        }
        if ((node = btree_leaf_node_right_neighbour(&as->as_area_btree, leaf))) {
                a = (as_area_t *) node->value[0];
                spinlock_lock(&a->lock);
                if (overlaps(va, size, a->base, a->pages * PAGE_SIZE)) {
                        spinlock_unlock(&a->lock);
                        return false;
                }
                spinlock_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;
        
                spinlock_lock(&a->lock);
                if (overlaps(va, size, a->base, a->pages * PAGE_SIZE)) {
                        spinlock_unlock(&a->lock);
                        return false;
                }
                spinlock_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;
}

/*
 * Address space related syscalls.
 */

/** Wrapper for as_area_create(). */
__native sys_as_area_create(__address address, size_t size, int flags)
{
        if (as_area_create(AS, flags, size, address, AS_AREA_ATTR_NONE))
                return (__native) address;
        else
                return (__native) -1;
}

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

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

/** Prepare task for accepting address space area from another task.
 *
 * @param uspace_accept_arg Accept structure passed from userspace.
 *
 * @return EPERM if the task ID encapsulated in @uspace_accept_arg references
 *         TASK. Otherwise zero is returned.
 */
__native sys_as_area_accept(as_area_acptsnd_arg_t *uspace_accept_arg)
{
        as_area_acptsnd_arg_t arg;
        int rc;
        
        rc = copy_from_uspace(&arg, uspace_accept_arg, sizeof(as_area_acptsnd_arg_t));
        if (rc != 0)
                return rc;
        
        if (!arg.size)
                return (__native) EPERM;
        
        if (arg.task_id == TASK->taskid) {
                /*
                 * Accepting from itself is not allowed.
                 */
                return (__native) EPERM;
        }
        
        memcpy(&TASK->accept_arg, &arg, sizeof(as_area_acptsnd_arg_t));
        
        return 0;
}

/** Wrapper for as_area_send. */
__native sys_as_area_send(as_area_acptsnd_arg_t *uspace_send_arg)
{
        as_area_acptsnd_arg_t arg;
        int rc;
        
        rc = copy_from_uspace(&arg, uspace_send_arg, sizeof(as_area_acptsnd_arg_t));
        if (rc != 0)
                return rc;

        if (!arg.size)
                return (__native) EPERM;
        
        if (arg.task_id == TASK->taskid) {
                /*
                 * Sending to itself is not allowed.
                 */
                return (__native) EPERM;
        }

        return (__native) as_area_send(arg.task_id, (__address) arg.base);
}