aio: checking for NULL instead of IS_ERR

[mirror_ubuntu-artful-kernel.git] / fs / aio.c

/*
 *      An async IO implementation for Linux
 *      Written by Benjamin LaHaise <bcrl@kvack.org>
 *
 *      Implements an efficient asynchronous io interface.
 *
 *      Copyright 2000, 2001, 2002 Red Hat, Inc.  All Rights Reserved.
 *
 *      See ../COPYING for licensing terms.
 */
#define pr_fmt(fmt) "%s: " fmt, __func__

#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/errno.h>
#include <linux/time.h>
#include <linux/aio_abi.h>
#include <linux/export.h>
#include <linux/syscalls.h>
#include <linux/backing-dev.h>
#include <linux/uio.h>

#include <linux/sched.h>
#include <linux/fs.h>
#include <linux/file.h>
#include <linux/mm.h>
#include <linux/mman.h>
#include <linux/mmu_context.h>
#include <linux/percpu.h>
#include <linux/slab.h>
#include <linux/timer.h>
#include <linux/aio.h>
#include <linux/highmem.h>
#include <linux/workqueue.h>
#include <linux/security.h>
#include <linux/eventfd.h>
#include <linux/blkdev.h>
#include <linux/compat.h>
#include <linux/migrate.h>
#include <linux/ramfs.h>
#include <linux/percpu-refcount.h>
#include <linux/mount.h>

#include <asm/kmap_types.h>
#include <asm/uaccess.h>

#include "internal.h"

#define AIO_RING_MAGIC                  0xa10a10a1
#define AIO_RING_COMPAT_FEATURES        1
#define AIO_RING_INCOMPAT_FEATURES      0
struct aio_ring {
        unsigned        id;     /* kernel internal index number */
        unsigned        nr;     /* number of io_events */
        unsigned        head;
        unsigned        tail;

        unsigned        magic;
        unsigned        compat_features;
        unsigned        incompat_features;
        unsigned        header_length;  /* size of aio_ring */


        struct io_event         io_events[0];
}; /* 128 bytes + ring size */

#define AIO_RING_PAGES  8

struct kioctx_table {
        struct rcu_head rcu;
        unsigned        nr;
        struct kioctx   *table[];
};

struct kioctx_cpu {
        unsigned                reqs_available;
};

struct kioctx {
        struct percpu_ref       users;
        atomic_t                dead;

        unsigned long           user_id;

        struct __percpu kioctx_cpu *cpu;

        /*
         * For percpu reqs_available, number of slots we move to/from global
         * counter at a time:
         */
        unsigned                req_batch;
        /*
         * This is what userspace passed to io_setup(), it's not used for
         * anything but counting against the global max_reqs quota.
         *
         * The real limit is nr_events - 1, which will be larger (see
         * aio_setup_ring())
         */
        unsigned                max_reqs;

        /* Size of ringbuffer, in units of struct io_event */
        unsigned                nr_events;

        unsigned long           mmap_base;
        unsigned long           mmap_size;

        struct page             **ring_pages;
        long                    nr_pages;

        struct rcu_head         rcu_head;
        struct work_struct      free_work;

        struct {
                /*
                 * This counts the number of available slots in the ringbuffer,
                 * so we avoid overflowing it: it's decremented (if positive)
                 * when allocating a kiocb and incremented when the resulting
                 * io_event is pulled off the ringbuffer.
                 *
                 * We batch accesses to it with a percpu version.
                 */
                atomic_t        reqs_available;
        } ____cacheline_aligned_in_smp;

        struct {
                spinlock_t      ctx_lock;
                struct list_head active_reqs;   /* used for cancellation */
        } ____cacheline_aligned_in_smp;

        struct {
                struct mutex    ring_lock;
                wait_queue_head_t wait;
        } ____cacheline_aligned_in_smp;

        struct {
                unsigned        tail;
                spinlock_t      completion_lock;
        } ____cacheline_aligned_in_smp;

        struct page             *internal_pages[AIO_RING_PAGES];
        struct file             *aio_ring_file;

        unsigned                id;
};

/*------ sysctl variables----*/
static DEFINE_SPINLOCK(aio_nr_lock);
unsigned long aio_nr;           /* current system wide number of aio requests */
unsigned long aio_max_nr = 0x10000; /* system wide maximum number of aio requests */
/*----end sysctl variables---*/

static struct kmem_cache        *kiocb_cachep;
static struct kmem_cache        *kioctx_cachep;

static struct vfsmount *aio_mnt;

static const struct file_operations aio_ring_fops;
static const struct address_space_operations aio_ctx_aops;

static struct file *aio_private_file(struct kioctx *ctx, loff_t nr_pages)
{
        struct qstr this = QSTR_INIT("[aio]", 5);
        struct file *file;
        struct path path;
        struct inode *inode = alloc_anon_inode(aio_mnt->mnt_sb);
        if (IS_ERR(inode))
                return ERR_CAST(inode);

        inode->i_mapping->a_ops = &aio_ctx_aops;
        inode->i_mapping->private_data = ctx;
        inode->i_size = PAGE_SIZE * nr_pages;

        path.dentry = d_alloc_pseudo(aio_mnt->mnt_sb, &this);
        if (!path.dentry) {
                iput(inode);
                return ERR_PTR(-ENOMEM);
        }
        path.mnt = mntget(aio_mnt);

        d_instantiate(path.dentry, inode);
        file = alloc_file(&path, FMODE_READ | FMODE_WRITE, &aio_ring_fops);
        if (IS_ERR(file)) {
                path_put(&path);
                return file;
        }

        file->f_flags = O_RDWR;
        file->private_data = ctx;
        return file;
}

static struct dentry *aio_mount(struct file_system_type *fs_type,
                                int flags, const char *dev_name, void *data)
{
        static const struct dentry_operations ops = {
                .d_dname        = simple_dname,
        };
        return mount_pseudo(fs_type, "aio:", NULL, &ops, 0xa10a10a1);
}

/* aio_setup
 *      Creates the slab caches used by the aio routines, panic on
 *      failure as this is done early during the boot sequence.
 */
static int __init aio_setup(void)
{
        static struct file_system_type aio_fs = {
                .name           = "aio",
                .mount          = aio_mount,
                .kill_sb        = kill_anon_super,
        };
        aio_mnt = kern_mount(&aio_fs);
        if (IS_ERR(aio_mnt))
                panic("Failed to create aio fs mount.");

        kiocb_cachep = KMEM_CACHE(kiocb, SLAB_HWCACHE_ALIGN|SLAB_PANIC);
        kioctx_cachep = KMEM_CACHE(kioctx,SLAB_HWCACHE_ALIGN|SLAB_PANIC);

        pr_debug("sizeof(struct page) = %zu\n", sizeof(struct page));

        return 0;
}
__initcall(aio_setup);

static void put_aio_ring_file(struct kioctx *ctx)
{
        struct file *aio_ring_file = ctx->aio_ring_file;
        if (aio_ring_file) {
                truncate_setsize(aio_ring_file->f_inode, 0);

                /* Prevent further access to the kioctx from migratepages */
                spin_lock(&aio_ring_file->f_inode->i_mapping->private_lock);
                aio_ring_file->f_inode->i_mapping->private_data = NULL;
                ctx->aio_ring_file = NULL;
                spin_unlock(&aio_ring_file->f_inode->i_mapping->private_lock);

                fput(aio_ring_file);
        }
}

static void aio_free_ring(struct kioctx *ctx)
{
        int i;

        for (i = 0; i < ctx->nr_pages; i++) {
                pr_debug("pid(%d) [%d] page->count=%d\n", current->pid, i,
                                page_count(ctx->ring_pages[i]));
                put_page(ctx->ring_pages[i]);
        }

        put_aio_ring_file(ctx);

        if (ctx->ring_pages && ctx->ring_pages != ctx->internal_pages)
                kfree(ctx->ring_pages);
}

static int aio_ring_mmap(struct file *file, struct vm_area_struct *vma)
{
        vma->vm_ops = &generic_file_vm_ops;
        return 0;
}

static const struct file_operations aio_ring_fops = {
        .mmap = aio_ring_mmap,
};

static int aio_set_page_dirty(struct page *page)
{
        return 0;
}

#if IS_ENABLED(CONFIG_MIGRATION)
static int aio_migratepage(struct address_space *mapping, struct page *new,
                        struct page *old, enum migrate_mode mode)
{
        struct kioctx *ctx;
        unsigned long flags;
        int rc;

        /* Writeback must be complete */
        BUG_ON(PageWriteback(old));
        put_page(old);

        rc = migrate_page_move_mapping(mapping, new, old, NULL, mode);
        if (rc != MIGRATEPAGE_SUCCESS) {
                get_page(old);
                return rc;
        }

        get_page(new);

        /* We can potentially race against kioctx teardown here.  Use the
         * address_space's private data lock to protect the mapping's
         * private_data.
         */
        spin_lock(&mapping->private_lock);
        ctx = mapping->private_data;
        if (ctx) {
                pgoff_t idx;
                spin_lock_irqsave(&ctx->completion_lock, flags);
                migrate_page_copy(new, old);
                idx = old->index;
                if (idx < (pgoff_t)ctx->nr_pages)
                        ctx->ring_pages[idx] = new;
                spin_unlock_irqrestore(&ctx->completion_lock, flags);
        } else
                rc = -EBUSY;
        spin_unlock(&mapping->private_lock);

        return rc;
}
#endif

static const struct address_space_operations aio_ctx_aops = {
        .set_page_dirty = aio_set_page_dirty,
#if IS_ENABLED(CONFIG_MIGRATION)
        .migratepage    = aio_migratepage,
#endif
};

static int aio_setup_ring(struct kioctx *ctx)
{
        struct aio_ring *ring;
        unsigned nr_events = ctx->max_reqs;
        struct mm_struct *mm = current->mm;
        unsigned long size, populate;
        int nr_pages;
        int i;
        struct file *file;

        /* Compensate for the ring buffer's head/tail overlap entry */
        nr_events += 2; /* 1 is required, 2 for good luck */

        size = sizeof(struct aio_ring);
        size += sizeof(struct io_event) * nr_events;

        nr_pages = PFN_UP(size);
        if (nr_pages < 0)
                return -EINVAL;

        file = aio_private_file(ctx, nr_pages);
        if (IS_ERR(file)) {
                ctx->aio_ring_file = NULL;
                return -EAGAIN;
        }

        for (i = 0; i < nr_pages; i++) {
                struct page *page;
                page = find_or_create_page(file->f_inode->i_mapping,
                                           i, GFP_HIGHUSER | __GFP_ZERO);
                if (!page)
                        break;
                pr_debug("pid(%d) page[%d]->count=%d\n",
                         current->pid, i, page_count(page));
                SetPageUptodate(page);
                SetPageDirty(page);
                unlock_page(page);
        }
        ctx->aio_ring_file = file;
        nr_events = (PAGE_SIZE * nr_pages - sizeof(struct aio_ring))
                        / sizeof(struct io_event);

        ctx->ring_pages = ctx->internal_pages;
        if (nr_pages > AIO_RING_PAGES) {
                ctx->ring_pages = kcalloc(nr_pages, sizeof(struct page *),
                                          GFP_KERNEL);
                if (!ctx->ring_pages)
                        return -ENOMEM;
        }

        ctx->mmap_size = nr_pages * PAGE_SIZE;
        pr_debug("attempting mmap of %lu bytes\n", ctx->mmap_size);

        down_write(&mm->mmap_sem);
        ctx->mmap_base = do_mmap_pgoff(ctx->aio_ring_file, 0, ctx->mmap_size,
                                       PROT_READ | PROT_WRITE,
                                       MAP_SHARED | MAP_POPULATE, 0, &populate);
        if (IS_ERR((void *)ctx->mmap_base)) {
                up_write(&mm->mmap_sem);
                ctx->mmap_size = 0;
                aio_free_ring(ctx);
                return -EAGAIN;
        }

        pr_debug("mmap address: 0x%08lx\n", ctx->mmap_base);

        /* We must do this while still holding mmap_sem for write, as we
         * need to be protected against userspace attempting to mremap()
         * or munmap() the ring buffer.
         */
        ctx->nr_pages = get_user_pages(current, mm, ctx->mmap_base, nr_pages,
                                       1, 0, ctx->ring_pages, NULL);

        /* Dropping the reference here is safe as the page cache will hold
         * onto the pages for us.  It is also required so that page migration
         * can unmap the pages and get the right reference count.
         */
        for (i = 0; i < ctx->nr_pages; i++)
                put_page(ctx->ring_pages[i]);

        up_write(&mm->mmap_sem);

        if (unlikely(ctx->nr_pages != nr_pages)) {
                aio_free_ring(ctx);
                return -EAGAIN;
        }

        ctx->user_id = ctx->mmap_base;
        ctx->nr_events = nr_events; /* trusted copy */

        ring = kmap_atomic(ctx->ring_pages[0]);
        ring->nr = nr_events;   /* user copy */
        ring->id = ~0U;
        ring->head = ring->tail = 0;
        ring->magic = AIO_RING_MAGIC;
        ring->compat_features = AIO_RING_COMPAT_FEATURES;
        ring->incompat_features = AIO_RING_INCOMPAT_FEATURES;
        ring->header_length = sizeof(struct aio_ring);
        kunmap_atomic(ring);
        flush_dcache_page(ctx->ring_pages[0]);

        return 0;
}

#define AIO_EVENTS_PER_PAGE     (PAGE_SIZE / sizeof(struct io_event))
#define AIO_EVENTS_FIRST_PAGE   ((PAGE_SIZE - sizeof(struct aio_ring)) / sizeof(struct io_event))
#define AIO_EVENTS_OFFSET       (AIO_EVENTS_PER_PAGE - AIO_EVENTS_FIRST_PAGE)

void kiocb_set_cancel_fn(struct kiocb *req, kiocb_cancel_fn *cancel)
{
        struct kioctx *ctx = req->ki_ctx;
        unsigned long flags;

        spin_lock_irqsave(&ctx->ctx_lock, flags);

        if (!req->ki_list.next)
                list_add(&req->ki_list, &ctx->active_reqs);

        req->ki_cancel = cancel;

        spin_unlock_irqrestore(&ctx->ctx_lock, flags);
}
EXPORT_SYMBOL(kiocb_set_cancel_fn);

static int kiocb_cancel(struct kioctx *ctx, struct kiocb *kiocb)
{
        kiocb_cancel_fn *old, *cancel;

        /*
         * Don't want to set kiocb->ki_cancel = KIOCB_CANCELLED unless it
         * actually has a cancel function, hence the cmpxchg()
         */

        cancel = ACCESS_ONCE(kiocb->ki_cancel);
        do {
                if (!cancel || cancel == KIOCB_CANCELLED)
                        return -EINVAL;

                old = cancel;
                cancel = cmpxchg(&kiocb->ki_cancel, old, KIOCB_CANCELLED);
        } while (cancel != old);

        return cancel(kiocb);
}

static void free_ioctx_rcu(struct rcu_head *head)
{
        struct kioctx *ctx = container_of(head, struct kioctx, rcu_head);

        free_percpu(ctx->cpu);
        kmem_cache_free(kioctx_cachep, ctx);
}

/*
 * When this function runs, the kioctx has been removed from the "hash table"
 * and ctx->users has dropped to 0, so we know no more kiocbs can be submitted -
 * now it's safe to cancel any that need to be.
 */
static void free_ioctx(struct work_struct *work)
{
        struct kioctx *ctx = container_of(work, struct kioctx, free_work);
        struct aio_ring *ring;
        struct kiocb *req;
        unsigned cpu, avail;
        DEFINE_WAIT(wait);

        spin_lock_irq(&ctx->ctx_lock);

        while (!list_empty(&ctx->active_reqs)) {
                req = list_first_entry(&ctx->active_reqs,
                                       struct kiocb, ki_list);

                list_del_init(&req->ki_list);
                kiocb_cancel(ctx, req);
        }

        spin_unlock_irq(&ctx->ctx_lock);

        for_each_possible_cpu(cpu) {
                struct kioctx_cpu *kcpu = per_cpu_ptr(ctx->cpu, cpu);

                atomic_add(kcpu->reqs_available, &ctx->reqs_available);
                kcpu->reqs_available = 0;
        }

        while (1) {
                prepare_to_wait(&ctx->wait, &wait, TASK_UNINTERRUPTIBLE);

                ring = kmap_atomic(ctx->ring_pages[0]);
                avail = (ring->head <= ring->tail)
                         ? ring->tail - ring->head
                         : ctx->nr_events - ring->head + ring->tail;

                atomic_add(avail, &ctx->reqs_available);
                ring->head = ring->tail;
                kunmap_atomic(ring);

                if (atomic_read(&ctx->reqs_available) >= ctx->nr_events - 1)
                        break;

                schedule();
        }
        finish_wait(&ctx->wait, &wait);

        WARN_ON(atomic_read(&ctx->reqs_available) > ctx->nr_events - 1);

        aio_free_ring(ctx);

        pr_debug("freeing %p\n", ctx);

        /*
         * Here the call_rcu() is between the wait_event() for reqs_active to
         * hit 0, and freeing the ioctx.
         *
         * aio_complete() decrements reqs_active, but it has to touch the ioctx
         * after to issue a wakeup so we use rcu.
         */
        call_rcu(&ctx->rcu_head, free_ioctx_rcu);
}

static void free_ioctx_ref(struct percpu_ref *ref)
{
        struct kioctx *ctx = container_of(ref, struct kioctx, users);

        INIT_WORK(&ctx->free_work, free_ioctx);
        schedule_work(&ctx->free_work);
}

static int ioctx_add_table(struct kioctx *ctx, struct mm_struct *mm)
{
        unsigned i, new_nr;
        struct kioctx_table *table, *old;
        struct aio_ring *ring;

        spin_lock(&mm->ioctx_lock);
        rcu_read_lock();
        table = rcu_dereference(mm->ioctx_table);

        while (1) {
                if (table)
                        for (i = 0; i < table->nr; i++)
                                if (!table->table[i]) {
                                        ctx->id = i;
                                        table->table[i] = ctx;
                                        rcu_read_unlock();
                                        spin_unlock(&mm->ioctx_lock);

                                        ring = kmap_atomic(ctx->ring_pages[0]);
                                        ring->id = ctx->id;
                                        kunmap_atomic(ring);
                                        return 0;
                                }

                new_nr = (table ? table->nr : 1) * 4;

                rcu_read_unlock();
                spin_unlock(&mm->ioctx_lock);

                table = kzalloc(sizeof(*table) + sizeof(struct kioctx *) *
                                new_nr, GFP_KERNEL);
                if (!table)
                        return -ENOMEM;

                table->nr = new_nr;

                spin_lock(&mm->ioctx_lock);
                rcu_read_lock();
                old = rcu_dereference(mm->ioctx_table);

                if (!old) {
                        rcu_assign_pointer(mm->ioctx_table, table);
                } else if (table->nr > old->nr) {
                        memcpy(table->table, old->table,
                               old->nr * sizeof(struct kioctx *));

                        rcu_assign_pointer(mm->ioctx_table, table);
                        kfree_rcu(old, rcu);
                } else {
                        kfree(table);
                        table = old;
                }
        }
}

/* ioctx_alloc
 *      Allocates and initializes an ioctx.  Returns an ERR_PTR if it failed.
 */
static struct kioctx *ioctx_alloc(unsigned nr_events)
{
        struct mm_struct *mm = current->mm;
        struct kioctx *ctx;
        int err = -ENOMEM;

        /*
         * We keep track of the number of available ringbuffer slots, to prevent
         * overflow (reqs_available), and we also use percpu counters for this.
         *
         * So since up to half the slots might be on other cpu's percpu counters
         * and unavailable, double nr_events so userspace sees what they
         * expected: additionally, we move req_batch slots to/from percpu
         * counters at a time, so make sure that isn't 0:
         */
        nr_events = max(nr_events, num_possible_cpus() * 4);
        nr_events *= 2;

        /* Prevent overflows */
        if ((nr_events > (0x10000000U / sizeof(struct io_event))) ||
            (nr_events > (0x10000000U / sizeof(struct kiocb)))) {
                pr_debug("ENOMEM: nr_events too high\n");
                return ERR_PTR(-EINVAL);
        }

        if (!nr_events || (unsigned long)nr_events > (aio_max_nr * 2UL))
                return ERR_PTR(-EAGAIN);

        ctx = kmem_cache_zalloc(kioctx_cachep, GFP_KERNEL);
        if (!ctx)
                return ERR_PTR(-ENOMEM);

        ctx->max_reqs = nr_events;

        if (percpu_ref_init(&ctx->users, free_ioctx_ref))
                goto out_freectx;

        spin_lock_init(&ctx->ctx_lock);
        spin_lock_init(&ctx->completion_lock);
        mutex_init(&ctx->ring_lock);
        init_waitqueue_head(&ctx->wait);

        INIT_LIST_HEAD(&ctx->active_reqs);

        ctx->cpu = alloc_percpu(struct kioctx_cpu);
        if (!ctx->cpu)
                goto out_freeref;

        if (aio_setup_ring(ctx) < 0)
                goto out_freepcpu;

        atomic_set(&ctx->reqs_available, ctx->nr_events - 1);
        ctx->req_batch = (ctx->nr_events - 1) / (num_possible_cpus() * 4);
        if (ctx->req_batch < 1)
                ctx->req_batch = 1;

        /* limit the number of system wide aios */
        spin_lock(&aio_nr_lock);
        if (aio_nr + nr_events > (aio_max_nr * 2UL) ||
            aio_nr + nr_events < aio_nr) {
                spin_unlock(&aio_nr_lock);
                goto out_cleanup;
        }
        aio_nr += ctx->max_reqs;
        spin_unlock(&aio_nr_lock);

        percpu_ref_get(&ctx->users); /* io_setup() will drop this ref */

        err = ioctx_add_table(ctx, mm);
        if (err)
                goto out_cleanup_put;

        pr_debug("allocated ioctx %p[%ld]: mm=%p mask=0x%x\n",
                 ctx, ctx->user_id, mm, ctx->nr_events);
        return ctx;

out_cleanup_put:
        percpu_ref_put(&ctx->users);
out_cleanup:
        err = -EAGAIN;
        aio_free_ring(ctx);
out_freepcpu:
        free_percpu(ctx->cpu);
out_freeref:
        free_percpu(ctx->users.pcpu_count);
out_freectx:
        put_aio_ring_file(ctx);
        kmem_cache_free(kioctx_cachep, ctx);
        pr_debug("error allocating ioctx %d\n", err);
        return ERR_PTR(err);
}

/* kill_ioctx
 *      Cancels all outstanding aio requests on an aio context.  Used
 *      when the processes owning a context have all exited to encourage
 *      the rapid destruction of the kioctx.
 */
static void kill_ioctx(struct mm_struct *mm, struct kioctx *ctx)
{
        if (!atomic_xchg(&ctx->dead, 1)) {
                struct kioctx_table *table;

                spin_lock(&mm->ioctx_lock);
                rcu_read_lock();
                table = rcu_dereference(mm->ioctx_table);

                WARN_ON(ctx != table->table[ctx->id]);
                table->table[ctx->id] = NULL;
                rcu_read_unlock();
                spin_unlock(&mm->ioctx_lock);

                /* percpu_ref_kill() will do the necessary call_rcu() */
                wake_up_all(&ctx->wait);

                /*
                 * It'd be more correct to do this in free_ioctx(), after all
                 * the outstanding kiocbs have finished - but by then io_destroy
                 * has already returned, so io_setup() could potentially return
                 * -EAGAIN with no ioctxs actually in use (as far as userspace
                 *  could tell).
                 */
                spin_lock(&aio_nr_lock);
                BUG_ON(aio_nr - ctx->max_reqs > aio_nr);
                aio_nr -= ctx->max_reqs;
                spin_unlock(&aio_nr_lock);

                if (ctx->mmap_size)
                        vm_munmap(ctx->mmap_base, ctx->mmap_size);

                percpu_ref_kill(&ctx->users);
        }
}

/* wait_on_sync_kiocb:
 *      Waits on the given sync kiocb to complete.
 */
ssize_t wait_on_sync_kiocb(struct kiocb *req)
{
        while (!req->ki_ctx) {
                set_current_state(TASK_UNINTERRUPTIBLE);
                if (req->ki_ctx)
                        break;
                io_schedule();
        }
        __set_current_state(TASK_RUNNING);
        return req->ki_user_data;
}
EXPORT_SYMBOL(wait_on_sync_kiocb);

/*
 * exit_aio: called when the last user of mm goes away.  At this point, there is
 * no way for any new requests to be submited or any of the io_* syscalls to be
 * called on the context.
 *
 * There may be outstanding kiocbs, but free_ioctx() will explicitly wait on
 * them.
 */
void exit_aio(struct mm_struct *mm)
{
        struct kioctx_table *table;
        struct kioctx *ctx;
        unsigned i = 0;

        while (1) {
                rcu_read_lock();
                table = rcu_dereference(mm->ioctx_table);

                do {
                        if (!table || i >= table->nr) {
                                rcu_read_unlock();
                                rcu_assign_pointer(mm->ioctx_table, NULL);
                                if (table)
                                        kfree(table);
                                return;
                        }

                        ctx = table->table[i++];
                } while (!ctx);

                rcu_read_unlock();

                /*
                 * We don't need to bother with munmap() here -
                 * exit_mmap(mm) is coming and it'll unmap everything.
                 * Since aio_free_ring() uses non-zero ->mmap_size
                 * as indicator that it needs to unmap the area,
                 * just set it to 0; aio_free_ring() is the only
                 * place that uses ->mmap_size, so it's safe.
                 */
                ctx->mmap_size = 0;

                kill_ioctx(mm, ctx);
        }
}

static void put_reqs_available(struct kioctx *ctx, unsigned nr)
{
        struct kioctx_cpu *kcpu;

        preempt_disable();
        kcpu = this_cpu_ptr(ctx->cpu);

        kcpu->reqs_available += nr;
        while (kcpu->reqs_available >= ctx->req_batch * 2) {
                kcpu->reqs_available -= ctx->req_batch;
                atomic_add(ctx->req_batch, &ctx->reqs_available);
        }

        preempt_enable();
}

static bool get_reqs_available(struct kioctx *ctx)
{
        struct kioctx_cpu *kcpu;
        bool ret = false;

        preempt_disable();
        kcpu = this_cpu_ptr(ctx->cpu);

        if (!kcpu->reqs_available) {
                int old, avail = atomic_read(&ctx->reqs_available);

                do {
                        if (avail < ctx->req_batch)
                                goto out;

                        old = avail;
                        avail = atomic_cmpxchg(&ctx->reqs_available,
                                               avail, avail - ctx->req_batch);
                } while (avail != old);

                kcpu->reqs_available += ctx->req_batch;
        }

        ret = true;
        kcpu->reqs_available--;
out:
        preempt_enable();
        return ret;
}

/* aio_get_req
 *      Allocate a slot for an aio request.
 * Returns NULL if no requests are free.
 */
static inline struct kiocb *aio_get_req(struct kioctx *ctx)
{
        struct kiocb *req;

        if (!get_reqs_available(ctx))
                return NULL;

        req = kmem_cache_alloc(kiocb_cachep, GFP_KERNEL|__GFP_ZERO);
        if (unlikely(!req))
                goto out_put;

        req->ki_ctx = ctx;
        return req;
out_put:
        put_reqs_available(ctx, 1);
        return NULL;
}

static void kiocb_free(struct kiocb *req)
{
        if (req->ki_filp)
                fput(req->ki_filp);
        if (req->ki_eventfd != NULL)
                eventfd_ctx_put(req->ki_eventfd);
        kmem_cache_free(kiocb_cachep, req);
}

static struct kioctx *lookup_ioctx(unsigned long ctx_id)
{
        struct aio_ring __user *ring  = (void __user *)ctx_id;
        struct mm_struct *mm = current->mm;
        struct kioctx *ctx, *ret = NULL;
        struct kioctx_table *table;
        unsigned id;

        if (get_user(id, &ring->id))
                return NULL;

        rcu_read_lock();
        table = rcu_dereference(mm->ioctx_table);

        if (!table || id >= table->nr)
                goto out;

        ctx = table->table[id];
        if (ctx && ctx->user_id == ctx_id) {
                percpu_ref_get(&ctx->users);
                ret = ctx;
        }
out:
        rcu_read_unlock();
        return ret;
}

/* aio_complete
 *      Called when the io request on the given iocb is complete.
 */
void aio_complete(struct kiocb *iocb, long res, long res2)
{
        struct kioctx   *ctx = iocb->ki_ctx;
        struct aio_ring *ring;
        struct io_event *ev_page, *event;
        unsigned long   flags;
        unsigned tail, pos;

        /*
         * Special case handling for sync iocbs:
         *  - events go directly into the iocb for fast handling
         *  - the sync task with the iocb in its stack holds the single iocb
         *    ref, no other paths have a way to get another ref
         *  - the sync task helpfully left a reference to itself in the iocb
         */
        if (is_sync_kiocb(iocb)) {
                iocb->ki_user_data = res;
                smp_wmb();
                iocb->ki_ctx = ERR_PTR(-EXDEV);
                wake_up_process(iocb->ki_obj.tsk);
                return;
        }

        /*
         * Take rcu_read_lock() in case the kioctx is being destroyed, as we
         * need to issue a wakeup after incrementing reqs_available.
         */
        rcu_read_lock();

        if (iocb->ki_list.next) {
                unsigned long flags;

                spin_lock_irqsave(&ctx->ctx_lock, flags);
                list_del(&iocb->ki_list);
                spin_unlock_irqrestore(&ctx->ctx_lock, flags);
        }

        /*
         * Add a completion event to the ring buffer. Must be done holding
         * ctx->completion_lock to prevent other code from messing with the tail
         * pointer since we might be called from irq context.
         */
        spin_lock_irqsave(&ctx->completion_lock, flags);

        tail = ctx->tail;
        pos = tail + AIO_EVENTS_OFFSET;

        if (++tail >= ctx->nr_events)
                tail = 0;

        ev_page = kmap_atomic(ctx->ring_pages[pos / AIO_EVENTS_PER_PAGE]);
        event = ev_page + pos % AIO_EVENTS_PER_PAGE;

        event->obj = (u64)(unsigned long)iocb->ki_obj.user;
        event->data = iocb->ki_user_data;
        event->res = res;
        event->res2 = res2;

        kunmap_atomic(ev_page);
        flush_dcache_page(ctx->ring_pages[pos / AIO_EVENTS_PER_PAGE]);

        pr_debug("%p[%u]: %p: %p %Lx %lx %lx\n",
                 ctx, tail, iocb, iocb->ki_obj.user, iocb->ki_user_data,
                 res, res2);

        /* after flagging the request as done, we
         * must never even look at it again
         */
        smp_wmb();      /* make event visible before updating tail */

        ctx->tail = tail;

        ring = kmap_atomic(ctx->ring_pages[0]);
        ring->tail = tail;
        kunmap_atomic(ring);
        flush_dcache_page(ctx->ring_pages[0]);

        spin_unlock_irqrestore(&ctx->completion_lock, flags);

        pr_debug("added to ring %p at [%u]\n", iocb, tail);

        /*
         * Check if the user asked us to deliver the result through an
         * eventfd. The eventfd_signal() function is safe to be called
         * from IRQ context.
         */
        if (iocb->ki_eventfd != NULL)
                eventfd_signal(iocb->ki_eventfd, 1);

        /* everything turned out well, dispose of the aiocb. */
        kiocb_free(iocb);

        /*
         * We have to order our ring_info tail store above and test
         * of the wait list below outside the wait lock.  This is
         * like in wake_up_bit() where clearing a bit has to be
         * ordered with the unlocked test.
         */
        smp_mb();

        if (waitqueue_active(&ctx->wait))
                wake_up(&ctx->wait);

        rcu_read_unlock();
}
EXPORT_SYMBOL(aio_complete);

/* aio_read_events
 *      Pull an event off of the ioctx's event ring.  Returns the number of
 *      events fetched
 */
static long aio_read_events_ring(struct kioctx *ctx,
                                 struct io_event __user *event, long nr)
{
        struct aio_ring *ring;
        unsigned head, tail, pos;
        long ret = 0;
        int copy_ret;

        mutex_lock(&ctx->ring_lock);

        ring = kmap_atomic(ctx->ring_pages[0]);
        head = ring->head;
        tail = ring->tail;
        kunmap_atomic(ring);

        pr_debug("h%u t%u m%u\n", head, tail, ctx->nr_events);

        if (head == tail)
                goto out;

        while (ret < nr) {
                long avail;
                struct io_event *ev;
                struct page *page;

                avail = (head <= tail ?  tail : ctx->nr_events) - head;
                if (head == tail)
                        break;

                avail = min(avail, nr - ret);
                avail = min_t(long, avail, AIO_EVENTS_PER_PAGE -
                            ((head + AIO_EVENTS_OFFSET) % AIO_EVENTS_PER_PAGE));

                pos = head + AIO_EVENTS_OFFSET;
                page = ctx->ring_pages[pos / AIO_EVENTS_PER_PAGE];
                pos %= AIO_EVENTS_PER_PAGE;

                ev = kmap(page);
                copy_ret = copy_to_user(event + ret, ev + pos,
                                        sizeof(*ev) * avail);
                kunmap(page);

                if (unlikely(copy_ret)) {
                        ret = -EFAULT;
                        goto out;
                }

                ret += avail;
                head += avail;
                head %= ctx->nr_events;
        }

        ring = kmap_atomic(ctx->ring_pages[0]);
        ring->head = head;
        kunmap_atomic(ring);
        flush_dcache_page(ctx->ring_pages[0]);

        pr_debug("%li  h%u t%u\n", ret, head, tail);

        put_reqs_available(ctx, ret);
out:
        mutex_unlock(&ctx->ring_lock);

        return ret;
}

static bool aio_read_events(struct kioctx *ctx, long min_nr, long nr,
                            struct io_event __user *event, long *i)
{
        long ret = aio_read_events_ring(ctx, event + *i, nr - *i);

        if (ret > 0)
                *i += ret;

        if (unlikely(atomic_read(&ctx->dead)))
                ret = -EINVAL;

        if (!*i)
                *i = ret;

        return ret < 0 || *i >= min_nr;
}

static long read_events(struct kioctx *ctx, long min_nr, long nr,
                        struct io_event __user *event,
                        struct timespec __user *timeout)
{
        ktime_t until = { .tv64 = KTIME_MAX };
        long ret = 0;

        if (timeout) {
                struct timespec ts;

                if (unlikely(copy_from_user(&ts, timeout, sizeof(ts))))
                        return -EFAULT;

                until = timespec_to_ktime(ts);
        }

        /*
         * Note that aio_read_events() is being called as the conditional - i.e.
         * we're calling it after prepare_to_wait() has set task state to
         * TASK_INTERRUPTIBLE.
         *
         * But aio_read_events() can block, and if it blocks it's going to flip
         * the task state back to TASK_RUNNING.
         *
         * This should be ok, provided it doesn't flip the state back to
         * TASK_RUNNING and return 0 too much - that causes us to spin. That
         * will only happen if the mutex_lock() call blocks, and we then find
         * the ringbuffer empty. So in practice we should be ok, but it's
         * something to be aware of when touching this code.
         */
        wait_event_interruptible_hrtimeout(ctx->wait,
                        aio_read_events(ctx, min_nr, nr, event, &ret), until);

        if (!ret && signal_pending(current))
                ret = -EINTR;

        return ret;
}

/* sys_io_setup:
 *      Create an aio_context capable of receiving at least nr_events.
 *      ctxp must not point to an aio_context that already exists, and
 *      must be initialized to 0 prior to the call.  On successful
 *      creation of the aio_context, *ctxp is filled in with the resulting 
 *      handle.  May fail with -EINVAL if *ctxp is not initialized,
 *      if the specified nr_events exceeds internal limits.  May fail 
 *      with -EAGAIN if the specified nr_events exceeds the user's limit 
 *      of available events.  May fail with -ENOMEM if insufficient kernel
 *      resources are available.  May fail with -EFAULT if an invalid
 *      pointer is passed for ctxp.  Will fail with -ENOSYS if not
 *      implemented.
 */
SYSCALL_DEFINE2(io_setup, unsigned, nr_events, aio_context_t __user *, ctxp)
{
        struct kioctx *ioctx = NULL;
        unsigned long ctx;
        long ret;

        ret = get_user(ctx, ctxp);
        if (unlikely(ret))
                goto out;

        ret = -EINVAL;
        if (unlikely(ctx || nr_events == 0)) {
                pr_debug("EINVAL: io_setup: ctx %lu nr_events %u\n",
                         ctx, nr_events);
                goto out;
        }

        ioctx = ioctx_alloc(nr_events);
        ret = PTR_ERR(ioctx);
        if (!IS_ERR(ioctx)) {
                ret = put_user(ioctx->user_id, ctxp);
                if (ret)
                        kill_ioctx(current->mm, ioctx);
                percpu_ref_put(&ioctx->users);
        }

out:
        return ret;
}

/* sys_io_destroy:
 *      Destroy the aio_context specified.  May cancel any outstanding 
 *      AIOs and block on completion.  Will fail with -ENOSYS if not
 *      implemented.  May fail with -EINVAL if the context pointed to
 *      is invalid.
 */
SYSCALL_DEFINE1(io_destroy, aio_context_t, ctx)
{
        struct kioctx *ioctx = lookup_ioctx(ctx);
        if (likely(NULL != ioctx)) {
                kill_ioctx(current->mm, ioctx);
                percpu_ref_put(&ioctx->users);
                return 0;
        }
        pr_debug("EINVAL: io_destroy: invalid context id\n");
        return -EINVAL;
}

typedef ssize_t (aio_rw_op)(struct kiocb *, const struct iovec *,
                            unsigned long, loff_t);

static ssize_t aio_setup_vectored_rw(struct kiocb *kiocb,
                                     int rw, char __user *buf,
                                     unsigned long *nr_segs,
                                     struct iovec **iovec,
                                     bool compat)
{
        ssize_t ret;

        *nr_segs = kiocb->ki_nbytes;

#ifdef CONFIG_COMPAT
        if (compat)
                ret = compat_rw_copy_check_uvector(rw,
                                (struct compat_iovec __user *)buf,
                                *nr_segs, 1, *iovec, iovec);
        else
#endif
                ret = rw_copy_check_uvector(rw,
                                (struct iovec __user *)buf,
                                *nr_segs, 1, *iovec, iovec);
        if (ret < 0)
                return ret;

        /* ki_nbytes now reflect bytes instead of segs */
        kiocb->ki_nbytes = ret;
        return 0;
}

static ssize_t aio_setup_single_vector(struct kiocb *kiocb,
                                       int rw, char __user *buf,
                                       unsigned long *nr_segs,
                                       struct iovec *iovec)
{
        if (unlikely(!access_ok(!rw, buf, kiocb->ki_nbytes)))
                return -EFAULT;

        iovec->iov_base = buf;
        iovec->iov_len = kiocb->ki_nbytes;
        *nr_segs = 1;
        return 0;
}

/*
 * aio_setup_iocb:
 *      Performs the initial checks and aio retry method
 *      setup for the kiocb at the time of io submission.
 */
static ssize_t aio_run_iocb(struct kiocb *req, unsigned opcode,
                            char __user *buf, bool compat)
{
        struct file *file = req->ki_filp;
        ssize_t ret;
        unsigned long nr_segs;
        int rw;
        fmode_t mode;
        aio_rw_op *rw_op;
        struct iovec inline_vec, *iovec = &inline_vec;

        switch (opcode) {
        case IOCB_CMD_PREAD:
        case IOCB_CMD_PREADV:
                mode    = FMODE_READ;
                rw      = READ;
                rw_op   = file->f_op->aio_read;
                goto rw_common;

        case IOCB_CMD_PWRITE:
        case IOCB_CMD_PWRITEV:
                mode    = FMODE_WRITE;
                rw      = WRITE;
                rw_op   = file->f_op->aio_write;
                goto rw_common;
rw_common:
                if (unlikely(!(file->f_mode & mode)))
                        return -EBADF;

                if (!rw_op)
                        return -EINVAL;

                ret = (opcode == IOCB_CMD_PREADV ||
                       opcode == IOCB_CMD_PWRITEV)
                        ? aio_setup_vectored_rw(req, rw, buf, &nr_segs,
                                                &iovec, compat)
                        : aio_setup_single_vector(req, rw, buf, &nr_segs,
                                                  iovec);
                if (ret)
                        return ret;

                ret = rw_verify_area(rw, file, &req->ki_pos, req->ki_nbytes);
                if (ret < 0) {
                        if (iovec != &inline_vec)
                                kfree(iovec);
                        return ret;
                }

                req->ki_nbytes = ret;

                /* XXX: move/kill - rw_verify_area()? */
                /* This matches the pread()/pwrite() logic */
                if (req->ki_pos < 0) {
                        ret = -EINVAL;
                        break;
                }

                if (rw == WRITE)
                        file_start_write(file);

                ret = rw_op(req, iovec, nr_segs, req->ki_pos);

                if (rw == WRITE)
                        file_end_write(file);
                break;

        case IOCB_CMD_FDSYNC:
                if (!file->f_op->aio_fsync)
                        return -EINVAL;

                ret = file->f_op->aio_fsync(req, 1);
                break;

        case IOCB_CMD_FSYNC:
                if (!file->f_op->aio_fsync)
                        return -EINVAL;

                ret = file->f_op->aio_fsync(req, 0);
                break;

        default:
                pr_debug("EINVAL: no operation provided\n");
                return -EINVAL;
        }

        if (iovec != &inline_vec)
                kfree(iovec);

        if (ret != -EIOCBQUEUED) {
                /*
                 * There's no easy way to restart the syscall since other AIO's
                 * may be already running. Just fail this IO with EINTR.
                 */
                if (unlikely(ret == -ERESTARTSYS || ret == -ERESTARTNOINTR ||
                             ret == -ERESTARTNOHAND ||
                             ret == -ERESTART_RESTARTBLOCK))
                        ret = -EINTR;
                aio_complete(req, ret, 0);
        }

        return 0;
}

static int io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
                         struct iocb *iocb, bool compat)
{
        struct kiocb *req;
        ssize_t ret;

        /* enforce forwards compatibility on users */
        if (unlikely(iocb->aio_reserved1 || iocb->aio_reserved2)) {
                pr_debug("EINVAL: reserve field set\n");
                return -EINVAL;
        }

        /* prevent overflows */
        if (unlikely(
            (iocb->aio_buf != (unsigned long)iocb->aio_buf) ||
            (iocb->aio_nbytes != (size_t)iocb->aio_nbytes) ||
            ((ssize_t)iocb->aio_nbytes < 0)
           )) {
                pr_debug("EINVAL: io_submit: overflow check\n");
                return -EINVAL;
        }

        req = aio_get_req(ctx);
        if (unlikely(!req))
                return -EAGAIN;

        req->ki_filp = fget(iocb->aio_fildes);
        if (unlikely(!req->ki_filp)) {
                ret = -EBADF;
                goto out_put_req;
        }

        if (iocb->aio_flags & IOCB_FLAG_RESFD) {
                /*
                 * If the IOCB_FLAG_RESFD flag of aio_flags is set, get an
                 * instance of the file* now. The file descriptor must be
                 * an eventfd() fd, and will be signaled for each completed
                 * event using the eventfd_signal() function.
                 */
                req->ki_eventfd = eventfd_ctx_fdget((int) iocb->aio_resfd);
                if (IS_ERR(req->ki_eventfd)) {
                        ret = PTR_ERR(req->ki_eventfd);
                        req->ki_eventfd = NULL;
                        goto out_put_req;
                }
        }

        ret = put_user(KIOCB_KEY, &user_iocb->aio_key);
        if (unlikely(ret)) {
                pr_debug("EFAULT: aio_key\n");
                goto out_put_req;
        }

        req->ki_obj.user = user_iocb;
        req->ki_user_data = iocb->aio_data;
        req->ki_pos = iocb->aio_offset;
        req->ki_nbytes = iocb->aio_nbytes;

        ret = aio_run_iocb(req, iocb->aio_lio_opcode,
                           (char __user *)(unsigned long)iocb->aio_buf,
                           compat);
        if (ret)
                goto out_put_req;

        return 0;
out_put_req:
        put_reqs_available(ctx, 1);
        kiocb_free(req);
        return ret;
}

long do_io_submit(aio_context_t ctx_id, long nr,
                  struct iocb __user *__user *iocbpp, bool compat)
{
        struct kioctx *ctx;
        long ret = 0;
        int i = 0;
        struct blk_plug plug;

        if (unlikely(nr < 0))
                return -EINVAL;

        if (unlikely(nr > LONG_MAX/sizeof(*iocbpp)))
                nr = LONG_MAX/sizeof(*iocbpp);

        if (unlikely(!access_ok(VERIFY_READ, iocbpp, (nr*sizeof(*iocbpp)))))
                return -EFAULT;

        ctx = lookup_ioctx(ctx_id);
        if (unlikely(!ctx)) {
                pr_debug("EINVAL: invalid context id\n");
                return -EINVAL;
        }

        blk_start_plug(&plug);

        /*
         * AKPM: should this return a partial result if some of the IOs were
         * successfully submitted?
         */
        for (i=0; i<nr; i++) {
                struct iocb __user *user_iocb;
                struct iocb tmp;

                if (unlikely(__get_user(user_iocb, iocbpp + i))) {
                        ret = -EFAULT;
                        break;
                }

                if (unlikely(copy_from_user(&tmp, user_iocb, sizeof(tmp)))) {
                        ret = -EFAULT;
                        break;
                }

                ret = io_submit_one(ctx, user_iocb, &tmp, compat);
                if (ret)
                        break;
        }
        blk_finish_plug(&plug);

        percpu_ref_put(&ctx->users);
        return i ? i : ret;
}

/* sys_io_submit:
 *      Queue the nr iocbs pointed to by iocbpp for processing.  Returns
 *      the number of iocbs queued.  May return -EINVAL if the aio_context
 *      specified by ctx_id is invalid, if nr is < 0, if the iocb at
 *      *iocbpp[0] is not properly initialized, if the operation specified
 *      is invalid for the file descriptor in the iocb.  May fail with
 *      -EFAULT if any of the data structures point to invalid data.  May
 *      fail with -EBADF if the file descriptor specified in the first
 *      iocb is invalid.  May fail with -EAGAIN if insufficient resources
 *      are available to queue any iocbs.  Will return 0 if nr is 0.  Will
 *      fail with -ENOSYS if not implemented.
 */
SYSCALL_DEFINE3(io_submit, aio_context_t, ctx_id, long, nr,
                struct iocb __user * __user *, iocbpp)
{
        return do_io_submit(ctx_id, nr, iocbpp, 0);
}

/* lookup_kiocb
 *      Finds a given iocb for cancellation.
 */
static struct kiocb *lookup_kiocb(struct kioctx *ctx, struct iocb __user *iocb,
                                  u32 key)
{
        struct list_head *pos;

        assert_spin_locked(&ctx->ctx_lock);

        if (key != KIOCB_KEY)
                return NULL;

        /* TODO: use a hash or array, this sucks. */
        list_for_each(pos, &ctx->active_reqs) {
                struct kiocb *kiocb = list_kiocb(pos);
                if (kiocb->ki_obj.user == iocb)
                        return kiocb;
        }
        return NULL;
}

/* sys_io_cancel:
 *      Attempts to cancel an iocb previously passed to io_submit.  If
 *      the operation is successfully cancelled, the resulting event is
 *      copied into the memory pointed to by result without being placed
 *      into the completion queue and 0 is returned.  May fail with
 *      -EFAULT if any of the data structures pointed to are invalid.
 *      May fail with -EINVAL if aio_context specified by ctx_id is
 *      invalid.  May fail with -EAGAIN if the iocb specified was not
 *      cancelled.  Will fail with -ENOSYS if not implemented.
 */
SYSCALL_DEFINE3(io_cancel, aio_context_t, ctx_id, struct iocb __user *, iocb,
                struct io_event __user *, result)
{
        struct kioctx *ctx;
        struct kiocb *kiocb;
        u32 key;
        int ret;

        ret = get_user(key, &iocb->aio_key);
        if (unlikely(ret))
                return -EFAULT;

        ctx = lookup_ioctx(ctx_id);
        if (unlikely(!ctx))
                return -EINVAL;

        spin_lock_irq(&ctx->ctx_lock);

        kiocb = lookup_kiocb(ctx, iocb, key);
        if (kiocb)
                ret = kiocb_cancel(ctx, kiocb);
        else
                ret = -EINVAL;

        spin_unlock_irq(&ctx->ctx_lock);

        if (!ret) {
                /*
                 * The result argument is no longer used - the io_event is
                 * always delivered via the ring buffer. -EINPROGRESS indicates
                 * cancellation is progress:
                 */
                ret = -EINPROGRESS;
        }

        percpu_ref_put(&ctx->users);

        return ret;
}

/* io_getevents:
 *      Attempts to read at least min_nr events and up to nr events from
 *      the completion queue for the aio_context specified by ctx_id. If
 *      it succeeds, the number of read events is returned. May fail with
 *      -EINVAL if ctx_id is invalid, if min_nr is out of range, if nr is
 *      out of range, if timeout is out of range.  May fail with -EFAULT
 *      if any of the memory specified is invalid.  May return 0 or
 *      < min_nr if the timeout specified by timeout has elapsed
 *      before sufficient events are available, where timeout == NULL
 *      specifies an infinite timeout. Note that the timeout pointed to by
 *      timeout is relative.  Will fail with -ENOSYS if not implemented.
 */
SYSCALL_DEFINE5(io_getevents, aio_context_t, ctx_id,
                long, min_nr,
                long, nr,
                struct io_event __user *, events,
                struct timespec __user *, timeout)
{
        struct kioctx *ioctx = lookup_ioctx(ctx_id);
        long ret = -EINVAL;

        if (likely(ioctx)) {
                if (likely(min_nr <= nr && min_nr >= 0))
                        ret = read_events(ioctx, min_nr, nr, events, timeout);
                percpu_ref_put(&ioctx->users);
        }
        return ret;
}