text: /*
   *	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;
  
  	struct percpu_ref	reqs;
  
  	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 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++) {
  		struct page *page;
  		pr_debug("pid(%d) [%d] page->count=%d\n", current->pid, i,
  				page_count(ctx->ring_pages[i]));
  		page = ctx->ring_pages[i];
  		if (!page)
  			continue;
  		ctx->ring_pages[i] = NULL;
  		put_page(page);
  	}
  
  	put_aio_ring_file(ctx);
  
  	if (ctx->ring_pages && ctx->ring_pages != ctx->internal_pages) {
  		kfree(ctx->ring_pages);
  		ctx->ring_pages = NULL;
  	}
  }
  
  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;
  
  	rc = 0;
  
  	/* Make sure the old page hasn't already been changed */
  	spin_lock(&mapping->private_lock);
  	ctx = mapping->private_data;
  	if (ctx) {
  		pgoff_t idx;
  		spin_lock_irqsave(&ctx->completion_lock, flags);
  		idx = old->index;
  		if (idx < (pgoff_t)ctx->nr_pages) {
  			if (ctx->ring_pages[idx] != old)
  				rc = -EAGAIN;
  		} else
  			rc = -EINVAL;
  		spin_unlock_irqrestore(&ctx->completion_lock, flags);
  	} else
  		rc = -EINVAL;
  	spin_unlock(&mapping->private_lock);
  
  	if (rc != 0)
  		return rc;
  
  	/* Writeback must be complete */
  	BUG_ON(PageWriteback(old));
  	get_page(new);
  
  	rc = migrate_page_move_mapping(mapping, new, old, NULL, mode, 1);
  	if (rc != MIGRATEPAGE_SUCCESS) {
  		put_page(new);
  		return rc;
  	}
  
  	/* 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) {
  			/* And only do the move if things haven't changed */
  			if (ctx->ring_pages[idx] == old)
  				ctx->ring_pages[idx] = new;
  			else
  				rc = -EAGAIN;
  		} else
  			rc = -EINVAL;
  		spin_unlock_irqrestore(&ctx->completion_lock, flags);
  	} else
  		rc = -EBUSY;
  	spin_unlock(&mapping->private_lock);
  
  	if (rc == MIGRATEPAGE_SUCCESS)
  		put_page(old);
  	else
  		put_page(new);
  
  	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, unused;
  	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;
  	}
  
  	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) {
  			put_aio_ring_file(ctx);
  			return -ENOMEM;
  		}
  	}
  
  	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->ring_pages[i] = page;
  	}
  	ctx->nr_pages = i;
  
  	if (unlikely(i != nr_pages)) {
  		aio_free_ring(ctx);
  		return -EAGAIN;
  	}
  
  	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, 0, &unused);
  	up_write(&mm->mmap_sem);
  	if (IS_ERR((void *)ctx->mmap_base)) {
  		ctx->mmap_size = 0;
  		aio_free_ring(ctx);
  		return -EAGAIN;
  	}
  
  	pr_debug("mmap address: 0x%08lx\n", ctx->mmap_base);
  
  	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(struct work_struct *work)
  {
  	struct kioctx *ctx = container_of(work, struct kioctx, free_work);
  
  	pr_debug("freeing %p\n", ctx);
  
  	aio_free_ring(ctx);
  	free_percpu(ctx->cpu);
  	kmem_cache_free(kioctx_cachep, ctx);
  }
  
  static void free_ioctx_reqs(struct percpu_ref *ref)
  {
  	struct kioctx *ctx = container_of(ref, struct kioctx, reqs);
  
  	INIT_WORK(&ctx->free_work, free_ioctx);
  	schedule_work(&ctx->free_work);
  }
  
  /*
   * 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_users(struct percpu_ref *ref)
  {
  	struct kioctx *ctx = container_of(ref, struct kioctx, users);
  	struct kiocb *req;
  
  	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);
  
  	percpu_ref_kill(&ctx->reqs);
  	percpu_ref_put(&ctx->reqs);
  }
  
  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;
  		}
  	}
  }
  
  static void aio_nr_sub(unsigned nr)
  {
  	spin_lock(&aio_nr_lock);
  	if (WARN_ON(aio_nr - nr > aio_nr))
  		aio_nr = 0;
  	else
  		aio_nr -= nr;
  	spin_unlock(&aio_nr_lock);
  }
  
  /* 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_users))
  		goto err;
  
  	if (percpu_ref_init(&ctx->reqs, free_ioctx_reqs))
  		goto err;
  
  	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 err;
  
  	if (aio_setup_ring(ctx) < 0)
  		goto err;
  
  	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);
  		err = -EAGAIN;
  		goto err_ctx;
  	}
  	aio_nr += ctx->max_reqs;
  	spin_unlock(&aio_nr_lock);
  
  	percpu_ref_get(&ctx->users);	/* io_setup() will drop this ref */
  	percpu_ref_get(&ctx->reqs);	/* free_ioctx_users() will drop this */
  
  	err = ioctx_add_table(ctx, mm);
  	if (err)
  		goto err_cleanup;
  
  	pr_debug("allocated ioctx %p[%ld]: mm=%p mask=0x%x\n",
  		 ctx, ctx->user_id, mm, ctx->nr_events);
  	return ctx;
  
  err_cleanup:
  	aio_nr_sub(ctx->max_reqs);
  err_ctx:
  	aio_free_ring(ctx);
  err:
  	free_percpu(ctx->cpu);
  	free_percpu(ctx->reqs.pcpu_count);
  	free_percpu(ctx->users.pcpu_count);
  	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).
  		 */
  		aio_nr_sub(ctx->max_reqs);
  
  		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;
  
  	percpu_ref_get(&ctx->reqs);
  
  	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;
  	}
  
  	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);
  
  	percpu_ref_put(&ctx->reqs);
  }
  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);
  	percpu_ref_put(&ctx->reqs);
  	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;
  }
  