/*
   * fs/direct-io.c
   *
   * Copyright (C) 2002, Linus Torvalds.
   *
   * O_DIRECT
   *

   * 04Jul2002	Andrew Morton

   *		Initial version
   * 11Sep2002	janetinc@us.ibm.com
   * 		added readv/writev support.

   * 29Oct2002	Andrew Morton

   *		rewrote bio_add_page() support.
   * 30Oct2002	pbadari@us.ibm.com
   *		added support for non-aligned IO.
   * 06Nov2002	pbadari@us.ibm.com
   *		added asynchronous IO support.
   * 21Jul2003	nathans@sgi.com
   *		added IO completion notifier.
   */
  
  #include <linux/kernel.h>
  #include <linux/module.h>
  #include <linux/types.h>
  #include <linux/fs.h>
  #include <linux/mm.h>
  #include <linux/slab.h>
  #include <linux/highmem.h>
  #include <linux/pagemap.h>

  #include <linux/task_io_accounting_ops.h>

  #include <linux/bio.h>
  #include <linux/wait.h>
  #include <linux/err.h>
  #include <linux/blkdev.h>
  #include <linux/buffer_head.h>
  #include <linux/rwsem.h>
  #include <linux/uio.h>

  #include <linux/atomic.h>

  #include <linux/prefetch.h>

  
  /*
   * How many user pages to map in one call to get_user_pages().  This determines

   * the size of a structure in the slab cache

   */
  #define DIO_PAGES	64
  
  /*
   * This code generally works in units of "dio_blocks".  A dio_block is
   * somewhere between the hard sector size and the filesystem block size.  it
   * is determined on a per-invocation basis.   When talking to the filesystem
   * we need to convert dio_blocks to fs_blocks by scaling the dio_block quantity
   * down by dio->blkfactor.  Similarly, fs-blocksize quantities are converted
   * to bio_block quantities by shifting left by blkfactor.
   *
   * If blkfactor is zero then the user's request was aligned to the filesystem's
   * blocksize.

   */

  /* dio_state only used in the submission path */
  
  struct dio_submit {

  	struct bio *bio;		/* bio under assembly */

  	unsigned blkbits;		/* doesn't change */
  	unsigned blkfactor;		/* When we're using an alignment which
  					   is finer than the filesystem's soft
  					   blocksize, this specifies how much
  					   finer.  blkfactor=2 means 1/4-block
  					   alignment.  Does not change */
  	unsigned start_zero_done;	/* flag: sub-blocksize zeroing has
  					   been performed at the start of a
  					   write */
  	int pages_in_io;		/* approximate total IO pages */

  	sector_t block_in_file;		/* Current offset into the underlying
  					   file in dio_block units. */
  	unsigned blocks_available;	/* At block_in_file.  changes */

  	int reap_counter;		/* rate limit reaping */

  	sector_t final_block_in_request;/* doesn't change */

  	int boundary;			/* prev block is at a boundary */

  	get_block_t *get_block;		/* block mapping function */

  	dio_submit_t *submit_io;	/* IO submition function */

  	loff_t logical_offset_in_bio;	/* current first logical block in bio */

  	sector_t final_block_in_bio;	/* current final block in bio + 1 */
  	sector_t next_block_for_io;	/* next block to be put under IO,
  					   in dio_blocks units */

  
  	/*
  	 * Deferred addition of a page to the dio.  These variables are
  	 * private to dio_send_cur_page(), submit_page_section() and
  	 * dio_bio_add_page().
  	 */
  	struct page *cur_page;		/* The page */
  	unsigned cur_page_offset;	/* Offset into it, in bytes */
  	unsigned cur_page_len;		/* Nr of bytes at cur_page_offset */
  	sector_t cur_page_block;	/* Where it starts */

  	loff_t cur_page_fs_offset;	/* Offset in file */

  	struct iov_iter *iter;

  	/*
  	 * Page queue.  These variables belong to dio_refill_pages() and
  	 * dio_get_page().
  	 */

  	unsigned head;			/* next page to process */
  	unsigned tail;			/* last valid page + 1 */

  	size_t from, to;

  };
  
  /* dio_state communicated between submission path and end_io */
  struct dio {
  	int flags;			/* doesn't change */

  	int op;
  	int op_flags;

  	blk_qc_t bio_cookie;
  	struct block_device *bio_bdev;

  	struct inode *inode;

  	loff_t i_size;			/* i_size when submitted */
  	dio_iodone_t *end_io;		/* IO completion function */

  	void *private;			/* copy from map_bh.b_private */

  
  	/* BIO completion state */
  	spinlock_t bio_lock;		/* protects BIO fields below */

  	int page_errors;		/* errno from get_user_pages() */
  	int is_async;			/* is IO async ? */

  	bool defer_completion;		/* defer AIO completion to workqueue? */

  	bool should_dirty;		/* if pages should be dirtied */

  	int io_error;			/* IO error in completion path */

  	unsigned long refcount;		/* direct_io_worker() and bios */
  	struct bio *bio_list;		/* singly linked via bi_private */
  	struct task_struct *waiter;	/* waiting task (NULL if none) */
  
  	/* AIO related stuff */
  	struct kiocb *iocb;		/* kiocb */

  	ssize_t result;                 /* IO result */

  	/*
  	 * pages[] (and any fields placed after it) are not zeroed out at
  	 * allocation time.  Don't add new fields after pages[] unless you
  	 * wish that they not be zeroed.
  	 */

  	union {
  		struct page *pages[DIO_PAGES];	/* page buffer */
  		struct work_struct complete_work;/* deferred AIO completion */
  	};

  } ____cacheline_aligned_in_smp;
  
  static struct kmem_cache *dio_cache __read_mostly;

  
  /*
   * How many pages are in the queue?
   */

  static inline unsigned dio_pages_present(struct dio_submit *sdio)

  {

  	return sdio->tail - sdio->head;

  }
  
  /*
   * Go grab and pin some userspace pages.   Typically we'll get 64 at a time.
   */

  static inline int dio_refill_pages(struct dio *dio, struct dio_submit *sdio)

  {

  	ssize_t ret;

  	ret = iov_iter_get_pages(sdio->iter, dio->pages, LONG_MAX, DIO_PAGES,

  				&sdio->from);

  	if (ret < 0 && sdio->blocks_available && (dio->op == REQ_OP_WRITE)) {

  		struct page *page = ZERO_PAGE(0);

  		/*
  		 * A memory fault, but the filesystem has some outstanding
  		 * mapped blocks.  We need to use those blocks up to avoid
  		 * leaking stale data in the file.
  		 */
  		if (dio->page_errors == 0)
  			dio->page_errors = ret;

  		get_page(page);

  		dio->pages[0] = page;

  		sdio->head = 0;
  		sdio->tail = 1;

  		sdio->from = 0;
  		sdio->to = PAGE_SIZE;
  		return 0;

  	}
  
  	if (ret >= 0) {

  		iov_iter_advance(sdio->iter, ret);
  		ret += sdio->from;

  		sdio->head = 0;

  		sdio->tail = (ret + PAGE_SIZE - 1) / PAGE_SIZE;
  		sdio->to = ((ret - 1) & (PAGE_SIZE - 1)) + 1;
  		return 0;

  	}

  	return ret;	
  }
  
  /*
   * Get another userspace page.  Returns an ERR_PTR on error.  Pages are
   * buffered inside the dio so that we can call get_user_pages() against a
   * decent number of pages, less frequently.  To provide nicer use of the
   * L1 cache.
   */

  static inline struct page *dio_get_page(struct dio *dio,

  					struct dio_submit *sdio)

  {

  	if (dio_pages_present(sdio) == 0) {

  		int ret;

  		ret = dio_refill_pages(dio, sdio);

  		if (ret)
  			return ERR_PTR(ret);

  		BUG_ON(dio_pages_present(sdio) == 0);

  	}

  	return dio->pages[sdio->head];

  }

  /**
   * dio_complete() - called when all DIO BIO I/O has been completed
   * @offset: the byte offset in the file of the completed operation
   *

   * This drops i_dio_count, lets interested parties know that a DIO operation
   * has completed, and calculates the resulting return code for the operation.

   *
   * It lets the filesystem know if it registered an interest earlier via
   * get_block.  Pass the private field of the map buffer_head so that
   * filesystems can use it to hold additional state between get_block calls and
   * dio_complete.

   */

  static ssize_t dio_complete(struct dio *dio, ssize_t ret, bool is_async)

  {

  	loff_t offset = dio->iocb->ki_pos;

  	ssize_t transferred = 0;

  	/*
  	 * AIO submission can race with bio completion to get here while
  	 * expecting to have the last io completed by bio completion.
  	 * In that case -EIOCBQUEUED is in fact not an error we want
  	 * to preserve through this call.
  	 */
  	if (ret == -EIOCBQUEUED)
  		ret = 0;

  	if (dio->result) {
  		transferred = dio->result;
  
  		/* Check for short read case */

  		if ((dio->op == REQ_OP_READ) &&
  		    ((offset + transferred) > dio->i_size))

  			transferred = dio->i_size - offset;

  		/* ignore EFAULT if some IO has been done */
  		if (unlikely(ret == -EFAULT) && transferred)
  			ret = 0;

  	}

  	if (ret == 0)
  		ret = dio->page_errors;
  	if (ret == 0)
  		ret = dio->io_error;
  	if (ret == 0)
  		ret = transferred;

  	if (dio->end_io) {
  		int err;

  		// XXX: ki_pos??

  		err = dio->end_io(dio->iocb, offset, ret, dio->private);
  		if (err)
  			ret = err;
  	}

  	if (!(dio->flags & DIO_SKIP_DIO_COUNT))
  		inode_dio_end(dio->inode);

  	if (is_async) {

  		/*
  		 * generic_write_sync expects ki_pos to have been updated
  		 * already, but the submission path only does this for
  		 * synchronous I/O.
  		 */
  		dio->iocb->ki_pos += transferred;

  		if (dio->op == REQ_OP_WRITE)

  			ret = generic_write_sync(dio->iocb,  transferred);

  		dio->iocb->ki_complete(dio->iocb, ret, 0);

  	}

  	kmem_cache_free(dio_cache, dio);

  	return ret;

  }

  static void dio_aio_complete_work(struct work_struct *work)
  {
  	struct dio *dio = container_of(work, struct dio, complete_work);

  	dio_complete(dio, 0, true);

  }

  static int dio_bio_complete(struct dio *dio, struct bio *bio);

  /*
   * Asynchronous IO callback. 
   */

  static void dio_bio_end_aio(struct bio *bio)

  {
  	struct dio *dio = bio->bi_private;

  	unsigned long remaining;
  	unsigned long flags;

  	/* cleanup the bio */
  	dio_bio_complete(dio, bio);

  	spin_lock_irqsave(&dio->bio_lock, flags);
  	remaining = --dio->refcount;
  	if (remaining == 1 && dio->waiter)

  		wake_up_process(dio->waiter);

  	spin_unlock_irqrestore(&dio->bio_lock, flags);

  	if (remaining == 0) {

  		if (dio->result && dio->defer_completion) {
  			INIT_WORK(&dio->complete_work, dio_aio_complete_work);
  			queue_work(dio->inode->i_sb->s_dio_done_wq,
  				   &dio->complete_work);
  		} else {

  			dio_complete(dio, 0, true);

  		}

  	}

  }
  
  /*
   * The BIO completion handler simply queues the BIO up for the process-context
   * handler.
   *
   * During I/O bi_private points at the dio.  After I/O, bi_private is used to
   * implement a singly-linked list of completed BIOs, at dio->bio_list.
   */

  static void dio_bio_end_io(struct bio *bio)

  {
  	struct dio *dio = bio->bi_private;
  	unsigned long flags;

  	spin_lock_irqsave(&dio->bio_lock, flags);
  	bio->bi_private = dio->bio_list;
  	dio->bio_list = bio;

  	if (--dio->refcount == 1 && dio->waiter)

  		wake_up_process(dio->waiter);
  	spin_unlock_irqrestore(&dio->bio_lock, flags);

  }

  /**
   * dio_end_io - handle the end io action for the given bio
   * @bio: The direct io bio thats being completed
   * @error: Error if there was one
   *
   * This is meant to be called by any filesystem that uses their own dio_submit_t
   * so that the DIO specific endio actions are dealt with after the filesystem
   * has done it's completion work.
   */
  void dio_end_io(struct bio *bio, int error)
  {
  	struct dio *dio = bio->bi_private;
  
  	if (dio->is_async)

  		dio_bio_end_aio(bio);

  	else

  		dio_bio_end_io(bio);

  }
  EXPORT_SYMBOL_GPL(dio_end_io);

  static inline void

  dio_bio_alloc(struct dio *dio, struct dio_submit *sdio,
  	      struct block_device *bdev,
  	      sector_t first_sector, int nr_vecs)

  {
  	struct bio *bio;

  	/*
  	 * bio_alloc() is guaranteed to return a bio when called with

  	 * __GFP_RECLAIM and we request a valid number of vectors.

  	 */

  	bio = bio_alloc(GFP_KERNEL, nr_vecs);

  
  	bio->bi_bdev = bdev;

  	bio->bi_iter.bi_sector = first_sector;

  	bio_set_op_attrs(bio, dio->op, dio->op_flags);

  	if (dio->is_async)
  		bio->bi_end_io = dio_bio_end_aio;
  	else
  		bio->bi_end_io = dio_bio_end_io;

  	sdio->bio = bio;
  	sdio->logical_offset_in_bio = sdio->cur_page_fs_offset;

  }
  
  /*
   * In the AIO read case we speculatively dirty the pages before starting IO.
   * During IO completion, any of these pages which happen to have been written
   * back will be redirtied by bio_check_pages_dirty().

   *
   * bios hold a dio reference between submit_bio and ->end_io.

   */

  static inline void dio_bio_submit(struct dio *dio, struct dio_submit *sdio)

  {

  	struct bio *bio = sdio->bio;

  	unsigned long flags;

  
  	bio->bi_private = dio;

  
  	spin_lock_irqsave(&dio->bio_lock, flags);
  	dio->refcount++;
  	spin_unlock_irqrestore(&dio->bio_lock, flags);

  	if (dio->is_async && dio->op == REQ_OP_READ && dio->should_dirty)

  		bio_set_pages_dirty(bio);

  	dio->bio_bdev = bio->bi_bdev;

  	if (sdio->submit_io) {

  		sdio->submit_io(bio, dio->inode, sdio->logical_offset_in_bio);

  		dio->bio_cookie = BLK_QC_T_NONE;

  	} else

  		dio->bio_cookie = submit_bio(bio);

  	sdio->bio = NULL;
  	sdio->boundary = 0;
  	sdio->logical_offset_in_bio = 0;

  }
  
  /*
   * Release any resources in case of a failure
   */

  static inline void dio_cleanup(struct dio *dio, struct dio_submit *sdio)

  {

  	while (sdio->head < sdio->tail)

  		put_page(dio->pages[sdio->head++]);

  }
  
  /*

   * Wait for the next BIO to complete.  Remove it and return it.  NULL is
   * returned once all BIOs have been completed.  This must only be called once
   * all bios have been issued so that dio->refcount can only decrease.  This
   * requires that that the caller hold a reference on the dio.

   */
  static struct bio *dio_await_one(struct dio *dio)
  {
  	unsigned long flags;

  	struct bio *bio = NULL;

  
  	spin_lock_irqsave(&dio->bio_lock, flags);

  
  	/*
  	 * Wait as long as the list is empty and there are bios in flight.  bio
  	 * completion drops the count, maybe adds to the list, and wakes while
  	 * holding the bio_lock so we don't need set_current_state()'s barrier
  	 * and can call it after testing our condition.
  	 */
  	while (dio->refcount > 1 && dio->bio_list == NULL) {
  		__set_current_state(TASK_UNINTERRUPTIBLE);
  		dio->waiter = current;
  		spin_unlock_irqrestore(&dio->bio_lock, flags);

  		if (!(dio->iocb->ki_flags & IOCB_HIPRI) ||
  		    !blk_poll(bdev_get_queue(dio->bio_bdev), dio->bio_cookie))

  			io_schedule();

  		/* wake up sets us TASK_RUNNING */
  		spin_lock_irqsave(&dio->bio_lock, flags);
  		dio->waiter = NULL;

  	}

  	if (dio->bio_list) {
  		bio = dio->bio_list;
  		dio->bio_list = bio->bi_private;
  	}

  	spin_unlock_irqrestore(&dio->bio_lock, flags);
  	return bio;
  }
  
  /*
   * Process one completed BIO.  No locks are held.
   */
  static int dio_bio_complete(struct dio *dio, struct bio *bio)
  {

  	struct bio_vec *bvec;
  	unsigned i;

  	int err;

  	if (bio->bi_error)

  		dio->io_error = -EIO;

  	if (dio->is_async && dio->op == REQ_OP_READ && dio->should_dirty) {

  		err = bio->bi_error;

  		bio_check_pages_dirty(bio);	/* transfers ownership */

  	} else {

  		bio_for_each_segment_all(bvec, bio, i) {
  			struct page *page = bvec->bv_page;

  			if (dio->op == REQ_OP_READ && !PageCompound(page) &&

  					dio->should_dirty)

  				set_page_dirty_lock(page);

  			put_page(page);

  		}

  		err = bio->bi_error;

  		bio_put(bio);
  	}

  	return err;

  }
  
  /*

   * Wait on and process all in-flight BIOs.  This must only be called once
   * all bios have been issued so that the refcount can only decrease.
   * This just waits for all bios to make it through dio_bio_complete.  IO

   * errors are propagated through dio->io_error and should be propagated via

   * dio_complete().

   */

  static void dio_await_completion(struct dio *dio)

  {

  	struct bio *bio;
  	do {
  		bio = dio_await_one(dio);
  		if (bio)
  			dio_bio_complete(dio, bio);
  	} while (bio);

  }
  
  /*
   * A really large O_DIRECT read or write can generate a lot of BIOs.  So
   * to keep the memory consumption sane we periodically reap any completed BIOs
   * during the BIO generation phase.
   *
   * This also helps to limit the peak amount of pinned userspace memory.
   */

  static inline int dio_bio_reap(struct dio *dio, struct dio_submit *sdio)

  {
  	int ret = 0;

  	if (sdio->reap_counter++ >= 64) {

  		while (dio->bio_list) {
  			unsigned long flags;
  			struct bio *bio;
  			int ret2;
  
  			spin_lock_irqsave(&dio->bio_lock, flags);
  			bio = dio->bio_list;
  			dio->bio_list = bio->bi_private;
  			spin_unlock_irqrestore(&dio->bio_lock, flags);
  			ret2 = dio_bio_complete(dio, bio);
  			if (ret == 0)
  				ret = ret2;
  		}

  		sdio->reap_counter = 0;

  	}
  	return ret;
  }
  
  /*

   * Create workqueue for deferred direct IO completions. We allocate the
   * workqueue when it's first needed. This avoids creating workqueue for
   * filesystems that don't need it and also allows us to create the workqueue
   * late enough so the we can include s_id in the name of the workqueue.
   */
  static int sb_init_dio_done_wq(struct super_block *sb)
  {

  	struct workqueue_struct *old;

  	struct workqueue_struct *wq = alloc_workqueue("dio/%s",
  						      WQ_MEM_RECLAIM, 0,
  						      sb->s_id);
  	if (!wq)
  		return -ENOMEM;
  	/*
  	 * This has to be atomic as more DIOs can race to create the workqueue
  	 */

  	old = cmpxchg(&sb->s_dio_done_wq, NULL, wq);

  	/* Someone created workqueue before us? Free ours... */

  	if (old)

  		destroy_workqueue(wq);
  	return 0;
  }
  
  static int dio_set_defer_completion(struct dio *dio)
  {
  	struct super_block *sb = dio->inode->i_sb;
  
  	if (dio->defer_completion)
  		return 0;
  	dio->defer_completion = true;
  	if (!sb->s_dio_done_wq)
  		return sb_init_dio_done_wq(sb);
  	return 0;
  }
  
  /*

   * Call into the fs to map some more disk blocks.  We record the current number

   * of available blocks at sdio->blocks_available.  These are in units of the

   * fs blocksize, i_blocksize(inode).

   *
   * The fs is allowed to map lots of blocks at once.  If it wants to do that,
   * it uses the passed inode-relative block number as the file offset, as usual.
   *

   * get_block() is passed the number of i_blkbits-sized blocks which direct_io

   * has remaining to do.  The fs should not map more than this number of blocks.
   *
   * If the fs has mapped a lot of blocks, it should populate bh->b_size to
   * indicate how much contiguous disk space has been made available at
   * bh->b_blocknr.
   *
   * If *any* of the mapped blocks are new, then the fs must set buffer_new().
   * This isn't very efficient...
   *
   * In the case of filesystem holes: the fs may return an arbitrarily-large
   * hole by returning an appropriate value in b_size and by clearing
   * buffer_mapped().  However the direct-io code will only process holes one

   * block at a time - it will repeatedly call get_block() as it walks the hole.

   */

  static int get_more_blocks(struct dio *dio, struct dio_submit *sdio,
  			   struct buffer_head *map_bh)

  {
  	int ret;

  	sector_t fs_startblk;	/* Into file, in filesystem-sized blocks */

  	sector_t fs_endblk;	/* Into file, in filesystem-sized blocks */

  	unsigned long fs_count;	/* Number of filesystem-sized blocks */

  	int create;

  	unsigned int i_blkbits = sdio->blkbits + sdio->blkfactor;

  
  	/*
  	 * If there was a memory error and we've overwritten all the
  	 * mapped blocks then we can now return that memory error
  	 */
  	ret = dio->page_errors;
  	if (ret == 0) {

  		BUG_ON(sdio->block_in_file >= sdio->final_block_in_request);
  		fs_startblk = sdio->block_in_file >> sdio->blkfactor;

  		fs_endblk = (sdio->final_block_in_request - 1) >>
  					sdio->blkfactor;
  		fs_count = fs_endblk - fs_startblk + 1;

  		map_bh->b_state = 0;

  		map_bh->b_size = fs_count << i_blkbits;

  		/*

  		 * For writes that could fill holes inside i_size on a
  		 * DIO_SKIP_HOLES filesystem we forbid block creations: only
  		 * overwrites are permitted. We will return early to the caller
  		 * once we see an unmapped buffer head returned, and the caller
  		 * will fall back to buffered I/O.

  		 *
  		 * Otherwise the decision is left to the get_blocks method,
  		 * which may decide to handle it or also return an unmapped
  		 * buffer head.
  		 */

  		create = dio->op == REQ_OP_WRITE;

  		if (dio->flags & DIO_SKIP_HOLES) {

  			if (fs_startblk <= ((i_size_read(dio->inode) - 1) >>
  							i_blkbits))

  				create = 0;

  		}

  		ret = (*sdio->get_block)(dio->inode, fs_startblk,

  						map_bh, create);

  
  		/* Store for completion */
  		dio->private = map_bh->b_private;

  
  		if (ret == 0 && buffer_defer_completion(map_bh))
  			ret = dio_set_defer_completion(dio);

  	}
  	return ret;
  }
  
  /*
   * There is no bio.  Make one now.
   */

  static inline int dio_new_bio(struct dio *dio, struct dio_submit *sdio,
  		sector_t start_sector, struct buffer_head *map_bh)

  {
  	sector_t sector;
  	int ret, nr_pages;

  	ret = dio_bio_reap(dio, sdio);

  	if (ret)
  		goto out;

  	sector = start_sector << (sdio->blkbits - 9);

  	nr_pages = min(sdio->pages_in_io, BIO_MAX_PAGES);

  	BUG_ON(nr_pages <= 0);

  	dio_bio_alloc(dio, sdio, map_bh->b_bdev, sector, nr_pages);

  	sdio->boundary = 0;

  out:
  	return ret;
  }
  
  /*
   * Attempt to put the current chunk of 'cur_page' into the current BIO.  If
   * that was successful then update final_block_in_bio and take a ref against
   * the just-added page.
   *
   * Return zero on success.  Non-zero means the caller needs to start a new BIO.
   */

  static inline int dio_bio_add_page(struct dio_submit *sdio)

  {
  	int ret;

  	ret = bio_add_page(sdio->bio, sdio->cur_page,
  			sdio->cur_page_len, sdio->cur_page_offset);
  	if (ret == sdio->cur_page_len) {

  		/*
  		 * Decrement count only, if we are done with this page
  		 */

  		if ((sdio->cur_page_len + sdio->cur_page_offset) == PAGE_SIZE)
  			sdio->pages_in_io--;

  		get_page(sdio->cur_page);

  		sdio->final_block_in_bio = sdio->cur_page_block +
  			(sdio->cur_page_len >> sdio->blkbits);

  		ret = 0;
  	} else {
  		ret = 1;
  	}
  	return ret;
  }
  		
  /*
   * Put cur_page under IO.  The section of cur_page which is described by
   * cur_page_offset,cur_page_len is put into a BIO.  The section of cur_page
   * starts on-disk at cur_page_block.
   *
   * We take a ref against the page here (on behalf of its presence in the bio).
   *
   * The caller of this function is responsible for removing cur_page from the
   * dio, and for dropping the refcount which came from that presence.
   */

  static inline int dio_send_cur_page(struct dio *dio, struct dio_submit *sdio,
  		struct buffer_head *map_bh)

  {
  	int ret = 0;

  	if (sdio->bio) {
  		loff_t cur_offset = sdio->cur_page_fs_offset;
  		loff_t bio_next_offset = sdio->logical_offset_in_bio +

  			sdio->bio->bi_iter.bi_size;

  		/*

  		 * See whether this new request is contiguous with the old.
  		 *

  		 * Btrfs cannot handle having logically non-contiguous requests
  		 * submitted.  For example if you have

  		 *
  		 * Logical:  [0-4095][HOLE][8192-12287]

  		 * Physical: [0-4095]      [4096-8191]

  		 *
  		 * We cannot submit those pages together as one BIO.  So if our
  		 * current logical offset in the file does not equal what would
  		 * be the next logical offset in the bio, submit the bio we
  		 * have.

  		 */

  		if (sdio->final_block_in_bio != sdio->cur_page_block ||

  		    cur_offset != bio_next_offset)

  			dio_bio_submit(dio, sdio);

  	}

  	if (sdio->bio == NULL) {

  		ret = dio_new_bio(dio, sdio, sdio->cur_page_block, map_bh);

  		if (ret)
  			goto out;
  	}

  	if (dio_bio_add_page(sdio) != 0) {
  		dio_bio_submit(dio, sdio);

  		ret = dio_new_bio(dio, sdio, sdio->cur_page_block, map_bh);

  		if (ret == 0) {

  			ret = dio_bio_add_page(sdio);

  			BUG_ON(ret != 0);
  		}
  	}
  out:
  	return ret;
  }
  
  /*
   * An autonomous function to put a chunk of a page under deferred IO.
   *
   * The caller doesn't actually know (or care) whether this piece of page is in
   * a BIO, or is under IO or whatever.  We just take care of all possible 
   * situations here.  The separation between the logic of do_direct_IO() and
   * that of submit_page_section() is important for clarity.  Please don't break.
   *
   * The chunk of page starts on-disk at blocknr.
   *
   * We perform deferred IO, by recording the last-submitted page inside our
   * private part of the dio structure.  If possible, we just expand the IO
   * across that page here.
   *
   * If that doesn't work out then we put the old page into the bio and add this
   * page to the dio instead.
   */

  static inline int

  submit_page_section(struct dio *dio, struct dio_submit *sdio, struct page *page,

  		    unsigned offset, unsigned len, sector_t blocknr,
  		    struct buffer_head *map_bh)

  {
  	int ret = 0;

  	if (dio->op == REQ_OP_WRITE) {

  		/*
  		 * Read accounting is performed in submit_bio()
  		 */
  		task_io_account_write(len);
  	}

  	/*
  	 * Can we just grow the current page's presence in the dio?
  	 */

  	if (sdio->cur_page == page &&
  	    sdio->cur_page_offset + sdio->cur_page_len == offset &&
  	    sdio->cur_page_block +
  	    (sdio->cur_page_len >> sdio->blkbits) == blocknr) {
  		sdio->cur_page_len += len;

  		goto out;
  	}
  
  	/*
  	 * If there's a deferred page already there then send it.
  	 */

  	if (sdio->cur_page) {

  		ret = dio_send_cur_page(dio, sdio, map_bh);

  		put_page(sdio->cur_page);

  		sdio->cur_page = NULL;

  		if (ret)

  			return ret;

  	}

  	get_page(page);		/* It is in dio */

  	sdio->cur_page = page;
  	sdio->cur_page_offset = offset;
  	sdio->cur_page_len = len;
  	sdio->cur_page_block = blocknr;
  	sdio->cur_page_fs_offset = sdio->block_in_file << sdio->blkbits;

  out:

  	/*
  	 * If sdio->boundary then we want to schedule the IO now to
  	 * avoid metadata seeks.
  	 */
  	if (sdio->boundary) {
  		ret = dio_send_cur_page(dio, sdio, map_bh);

  		if (sdio->bio)
  			dio_bio_submit(dio, sdio);

  		put_page(sdio->cur_page);

  		sdio->cur_page = NULL;
  	}

  	return ret;
  }
  
  /*
   * Clean any dirty buffers in the blockdev mapping which alias newly-created
   * file blocks.  Only called for S_ISREG files - blockdevs do not set
   * buffer_new
   */

  static void clean_blockdev_aliases(struct dio *dio, struct buffer_head *map_bh)

  {
  	unsigned i;
  	unsigned nblocks;

  	nblocks = map_bh->b_size >> dio->inode->i_blkbits;

  
  	for (i = 0; i < nblocks; i++) {

  		unmap_underlying_metadata(map_bh->b_bdev,
  					  map_bh->b_blocknr + i);

  	}
  }
  
  /*
   * If we are not writing the entire block and get_block() allocated
   * the block for us, we need to fill-in the unused portion of the
   * block with zeros. This happens only if user-buffer, fileoffset or
   * io length is not filesystem block-size multiple.
   *
   * `end' is zero if we're doing the start of the IO, 1 at the end of the
   * IO.
   */

  static inline void dio_zero_block(struct dio *dio, struct dio_submit *sdio,
  		int end, struct buffer_head *map_bh)

  {
  	unsigned dio_blocks_per_fs_block;
  	unsigned this_chunk_blocks;	/* In dio_blocks */
  	unsigned this_chunk_bytes;
  	struct page *page;

  	sdio->start_zero_done = 1;

  	if (!sdio->blkfactor || !buffer_new(map_bh))

  		return;

  	dio_blocks_per_fs_block = 1 << sdio->blkfactor;
  	this_chunk_blocks = sdio->block_in_file & (dio_blocks_per_fs_block - 1);

  
  	if (!this_chunk_blocks)
  		return;
  
  	/*
  	 * We need to zero out part of an fs block.  It is either at the
  	 * beginning or the end of the fs block.
  	 */
  	if (end) 
  		this_chunk_blocks = dio_blocks_per_fs_block - this_chunk_blocks;

  	this_chunk_bytes = this_chunk_blocks << sdio->blkbits;

  	page = ZERO_PAGE(0);

  	if (submit_page_section(dio, sdio, page, 0, this_chunk_bytes,

  				sdio->next_block_for_io, map_bh))

  		return;

  	sdio->next_block_for_io += this_chunk_blocks;

  }
  
  /*
   * Walk the user pages, and the file, mapping blocks to disk and generating
   * a sequence of (page,offset,len,block) mappings.  These mappings are injected
   * into submit_page_section(), which takes care of the next stage of submission
   *
   * Direct IO against a blockdev is different from a file.  Because we can
   * happily perform page-sized but 512-byte aligned IOs.  It is important that
   * blockdev IO be able to have fine alignment and large sizes.
   *

   * So what we do is to permit the ->get_block function to populate bh.b_size

   * with the size of IO which is permitted at this offset and this i_blkbits.
   *
   * For best results, the blockdev should be set up with 512-byte i_blkbits and

   * it should set b_size to PAGE_SIZE or more inside get_block().  This gives

   * fine alignment but still allows this function to work in PAGE_SIZE units.
   */

  static int do_direct_IO(struct dio *dio, struct dio_submit *sdio,
  			struct buffer_head *map_bh)

  {

  	const unsigned blkbits = sdio->blkbits;

  	int ret = 0;

  	while (sdio->block_in_file < sdio->final_block_in_request) {

  		struct page *page;
  		size_t from, to;

  
  		page = dio_get_page(dio, sdio);

  		if (IS_ERR(page)) {
  			ret = PTR_ERR(page);
  			goto out;
  		}

  		from = sdio->head ? 0 : sdio->from;
  		to = (sdio->head == sdio->tail - 1) ? sdio->to : PAGE_SIZE;
  		sdio->head++;

  		while (from < to) {

  			unsigned this_chunk_bytes;	/* # of bytes mapped */
  			unsigned this_chunk_blocks;	/* # of blocks */
  			unsigned u;

  			if (sdio->blocks_available == 0) {

  				/*
  				 * Need to go and map some more disk
  				 */
  				unsigned long blkmask;
  				unsigned long dio_remainder;

  				ret = get_more_blocks(dio, sdio, map_bh);

  				if (ret) {

  					put_page(page);

  					goto out;
  				}
  				if (!buffer_mapped(map_bh))
  					goto do_holes;

  				sdio->blocks_available =
  						map_bh->b_size >> sdio->blkbits;
  				sdio->next_block_for_io =
  					map_bh->b_blocknr << sdio->blkfactor;

  				if (buffer_new(map_bh))

  					clean_blockdev_aliases(dio, map_bh);

  				if (!sdio->blkfactor)

  					goto do_holes;

  				blkmask = (1 << sdio->blkfactor) - 1;
  				dio_remainder = (sdio->block_in_file & blkmask);

  
  				/*
  				 * If we are at the start of IO and that IO
  				 * starts partway into a fs-block,
  				 * dio_remainder will be non-zero.  If the IO
  				 * is a read then we can simply advance the IO
  				 * cursor to the first block which is to be
  				 * read.  But if the IO is a write and the
  				 * block was newly allocated we cannot do that;
  				 * the start of the fs block must be zeroed out
  				 * on-disk
  				 */
  				if (!buffer_new(map_bh))

  					sdio->next_block_for_io += dio_remainder;
  				sdio->blocks_available -= dio_remainder;

  			}
  do_holes:
  			/* Handle holes */
  			if (!buffer_mapped(map_bh)) {

  				loff_t i_size_aligned;

  
  				/* AKPM: eargh, -ENOTBLK is a hack */

  				if (dio->op == REQ_OP_WRITE) {

  					put_page(page);

  					return -ENOTBLK;
  				}

  				/*
  				 * Be sure to account for a partial block as the
  				 * last block in the file
  				 */
  				i_size_aligned = ALIGN(i_size_read(dio->inode),
  							1 << blkbits);

  				if (sdio->block_in_file >=

  						i_size_aligned >> blkbits) {

  					/* We hit eof */

  					put_page(page);

  					goto out;
  				}

  				zero_user(page, from, 1 << blkbits);

  				sdio->block_in_file++;

  				from += 1 << blkbits;

  				dio->result += 1 << blkbits;

  				goto next_block;
  			}
  
  			/*
  			 * If we're performing IO which has an alignment which
  			 * is finer than the underlying fs, go check to see if
  			 * we must zero out the start of this block.
  			 */

  			if (unlikely(sdio->blkfactor && !sdio->start_zero_done))

  				dio_zero_block(dio, sdio, 0, map_bh);

  
  			/*
  			 * Work out, in this_chunk_blocks, how much disk we
  			 * can add to this page
  			 */

  			this_chunk_blocks = sdio->blocks_available;

  			u = (to - from) >> blkbits;

  			if (this_chunk_blocks > u)
  				this_chunk_blocks = u;

  			u = sdio->final_block_in_request - sdio->block_in_file;

  			if (this_chunk_blocks > u)
  				this_chunk_blocks = u;
  			this_chunk_bytes = this_chunk_blocks << blkbits;
  			BUG_ON(this_chunk_bytes == 0);

  			if (this_chunk_blocks == sdio->blocks_available)
  				sdio->boundary = buffer_boundary(map_bh);

  			ret = submit_page_section(dio, sdio, page,

  						  from,

  						  this_chunk_bytes,

  						  sdio->next_block_for_io,
  						  map_bh);

  			if (ret) {

  				put_page(page);

  				goto out;
  			}

  			sdio->next_block_for_io += this_chunk_blocks;

  			sdio->block_in_file += this_chunk_blocks;

  			from += this_chunk_bytes;
  			dio->result += this_chunk_bytes;

  			sdio->blocks_available -= this_chunk_blocks;

  next_block:

  			BUG_ON(sdio->block_in_file > sdio->final_block_in_request);
  			if (sdio->block_in_file == sdio->final_block_in_request)

  				break;
  		}
  
  		/* Drop the ref which was taken in get_user_pages() */

  		put_page(page);

  	}
  out:
  	return ret;
  }

  static inline int drop_refcount(struct dio *dio)

  {

  	int ret2;

  	unsigned long flags;

  	/*
  	 * Sync will always be dropping the final ref and completing the

  	 * operation.  AIO can if it was a broken operation described above or
  	 * in fact if all the bios race to complete before we get here.  In
  	 * that case dio_complete() translates the EIOCBQUEUED into the proper

  	 * return code that the caller will hand to ->complete().

  	 *
  	 * This is managed by the bio_lock instead of being an atomic_t so that
  	 * completion paths can drop their ref and use the remaining count to
  	 * decide to wake the submission path atomically.

  	 */

  	spin_lock_irqsave(&dio->bio_lock, flags);
  	ret2 = --dio->refcount;
  	spin_unlock_irqrestore(&dio->bio_lock, flags);

  	return ret2;

  }

  /*
   * This is a library function for use by filesystem drivers.
   *
   * The locking rules are governed by the flags parameter:
   *  - if the flags value contains DIO_LOCKING we use a fancy locking
   *    scheme for dumb filesystems.
   *    For writes this function is called under i_mutex and returns with
   *    i_mutex held, for reads, i_mutex is not held on entry, but it is
   *    taken and dropped again before returning.

   *  - if the flags value does NOT contain DIO_LOCKING we don't use any
   *    internal locking but rather rely on the filesystem to synchronize
   *    direct I/O reads/writes versus each other and truncate.

   *
   * To help with locking against truncate we incremented the i_dio_count
   * counter before starting direct I/O, and decrement it once we are done.
   * Truncate can wait for it to reach zero to provide exclusion.  It is
   * expected that filesystem provide exclusion between new direct I/O
   * and truncates.  For DIO_LOCKING filesystems this is done by i_mutex,
   * but other filesystems need to take care of this on their own.

   *
   * NOTE: if you pass "sdio" to anything by pointer make sure that function
   * is always inlined. Otherwise gcc is unable to split the structure into
   * individual fields and will generate much worse code. This is important
   * for the whole file.

   */

  static inline ssize_t

  do_blockdev_direct_IO(struct kiocb *iocb, struct inode *inode,
  		      struct block_device *bdev, struct iov_iter *iter,

  		      get_block_t get_block, dio_iodone_t end_io,

  		      dio_submit_t submit_io, int flags)

  {

  	unsigned i_blkbits = ACCESS_ONCE(inode->i_blkbits);
  	unsigned blkbits = i_blkbits;

  	unsigned blocksize_mask = (1 << blkbits) - 1;
  	ssize_t retval = -EINVAL;

  	size_t count = iov_iter_count(iter);

  	loff_t offset = iocb->ki_pos;

  	loff_t end = offset + count;

  	struct dio *dio;

  	struct dio_submit sdio = { 0, };

  	struct buffer_head map_bh = { 0, };

  	struct blk_plug plug;

  	unsigned long align = offset | iov_iter_alignment(iter);

  	/*
  	 * Avoid references to bdev if not absolutely needed to give
  	 * the early prefetch in the caller enough time.
  	 */

  	if (align & blocksize_mask) {

  		if (bdev)

  			blkbits = blksize_bits(bdev_logical_block_size(bdev));

  		blocksize_mask = (1 << blkbits) - 1;

  		if (align & blocksize_mask)

  			goto out;
  	}

  	/* watch out for a 0 len io from a tricksy fs */

  	if (iov_iter_rw(iter) == READ && !iov_iter_count(iter))

  		return 0;

  	dio = kmem_cache_alloc(dio_cache, GFP_KERNEL);

  	retval = -ENOMEM;
  	if (!dio)
  		goto out;

  	/*
  	 * Believe it or not, zeroing out the page array caused a .5%
  	 * performance regression in a database benchmark.  So, we take
  	 * care to only zero out what's needed.
  	 */
  	memset(dio, 0, offsetof(struct dio, pages));

  	dio->flags = flags;
  	if (dio->flags & DIO_LOCKING) {

  		if (iov_iter_rw(iter) == READ) {

  			struct address_space *mapping =
  					iocb->ki_filp->f_mapping;

  			/* will be released by direct_io_worker */

  			inode_lock(inode);

  
  			retval = filemap_write_and_wait_range(mapping, offset,
  							      end - 1);
  			if (retval) {

  				inode_unlock(inode);

  				kmem_cache_free(dio_cache, dio);

  				goto out;
  			}

  		}

  	}

  	/* Once we sampled i_size check for reads beyond EOF */
  	dio->i_size = i_size_read(inode);
  	if (iov_iter_rw(iter) == READ && offset >= dio->i_size) {
  		if (dio->flags & DIO_LOCKING)

  			inode_unlock(inode);

  		kmem_cache_free(dio_cache, dio);

  		retval = 0;

  		goto out;
  	}

  	/*

  	 * For file extending writes updating i_size before data writeouts
  	 * complete can expose uninitialized blocks in dumb filesystems.
  	 * In that case we need to wait for I/O completion even if asked
  	 * for an asynchronous write.

  	 */

  	if (is_sync_kiocb(iocb))
  		dio->is_async = false;
  	else if (!(dio->flags & DIO_ASYNC_EXTEND) &&

  		 iov_iter_rw(iter) == WRITE && end > i_size_read(inode))

  		dio->is_async = false;
  	else
  		dio->is_async = true;

  	dio->inode = inode;

  	if (iov_iter_rw(iter) == WRITE) {
  		dio->op = REQ_OP_WRITE;
  		dio->op_flags = WRITE_ODIRECT;
  	} else {
  		dio->op = REQ_OP_READ;
  	}

  
  	/*
  	 * For AIO O_(D)SYNC writes we need to defer completions to a workqueue
  	 * so that we can call ->fsync.
  	 */

  	if (dio->is_async && iov_iter_rw(iter) == WRITE &&

  	    ((iocb->ki_filp->f_flags & O_DSYNC) ||
  	     IS_SYNC(iocb->ki_filp->f_mapping->host))) {
  		retval = dio_set_defer_completion(dio);
  		if (retval) {
  			/*
  			 * We grab i_mutex only for reads so we don't have
  			 * to release it here
  			 */
  			kmem_cache_free(dio_cache, dio);
  			goto out;
  		}
  	}
  
  	/*
  	 * Will be decremented at I/O completion time.
  	 */

  	if (!(dio->flags & DIO_SKIP_DIO_COUNT))
  		inode_dio_begin(inode);

  
  	retval = 0;

  	sdio.blkbits = blkbits;

  	sdio.blkfactor = i_blkbits - blkbits;

  	sdio.block_in_file = offset >> blkbits;
  
  	sdio.get_block = get_block;
  	dio->end_io = end_io;
  	sdio.submit_io = submit_io;
  	sdio.final_block_in_bio = -1;
  	sdio.next_block_for_io = -1;
  
  	dio->iocb = iocb;

  
  	spin_lock_init(&dio->bio_lock);
  	dio->refcount = 1;

  	dio->should_dirty = (iter->type == ITER_IOVEC);

  	sdio.iter = iter;
  	sdio.final_block_in_request =
  		(offset + iov_iter_count(iter)) >> blkbits;

  	/*
  	 * In case of non-aligned buffers, we may need 2 more
  	 * pages since we need to zero out first and last block.
  	 */
  	if (unlikely(sdio.blkfactor))
  		sdio.pages_in_io = 2;

  	sdio.pages_in_io += iov_iter_npages(iter, INT_MAX);

  	blk_start_plug(&plug);

  	retval = do_direct_IO(dio, &sdio, &map_bh);
  	if (retval)
  		dio_cleanup(dio, &sdio);

  
  	if (retval == -ENOTBLK) {
  		/*
  		 * The remaining part of the request will be
  		 * be handled by buffered I/O when we return
  		 */
  		retval = 0;
  	}
  	/*
  	 * There may be some unwritten disk at the end of a part-written
  	 * fs-block-sized block.  Go zero that now.
  	 */
  	dio_zero_block(dio, &sdio, 1, &map_bh);
  
  	if (sdio.cur_page) {
  		ssize_t ret2;
  
  		ret2 = dio_send_cur_page(dio, &sdio, &map_bh);
  		if (retval == 0)
  			retval = ret2;

  		put_page(sdio.cur_page);

  		sdio.cur_page = NULL;
  	}
  	if (sdio.bio)
  		dio_bio_submit(dio, &sdio);

  	blk_finish_plug(&plug);

  	/*
  	 * It is possible that, we return short IO due to end of file.
  	 * In that case, we need to release all the pages we got hold on.
  	 */
  	dio_cleanup(dio, &sdio);
  
  	/*
  	 * All block lookups have been performed. For READ requests
  	 * we can let i_mutex go now that its achieved its purpose
  	 * of protecting us from looking up uninitialized blocks.
  	 */

  	if (iov_iter_rw(iter) == READ && (dio->flags & DIO_LOCKING))

  		inode_unlock(dio->inode);

  
  	/*
  	 * The only time we want to leave bios in flight is when a successful
  	 * partial aio read or full aio write have been setup.  In that case
  	 * bio completion will call aio_complete.  The only time it's safe to
  	 * call aio_complete is when we return -EIOCBQUEUED, so we key on that.
  	 * This had *better* be the only place that raises -EIOCBQUEUED.
  	 */
  	BUG_ON(retval == -EIOCBQUEUED);
  	if (dio->is_async && retval == 0 && dio->result &&

  	    (iov_iter_rw(iter) == READ || dio->result == count))

  		retval = -EIOCBQUEUED;

  	else

  		dio_await_completion(dio);
  
  	if (drop_refcount(dio) == 0) {

  		retval = dio_complete(dio, retval, false);

  	} else
  		BUG_ON(retval != -EIOCBQUEUED);

  out:
  	return retval;
  }

  ssize_t __blockdev_direct_IO(struct kiocb *iocb, struct inode *inode,
  			     struct block_device *bdev, struct iov_iter *iter,

  			     get_block_t get_block,

  			     dio_iodone_t end_io, dio_submit_t submit_io,
  			     int flags)

  {
  	/*
  	 * The block device state is needed in the end to finally
  	 * submit everything.  Since it's likely to be cache cold
  	 * prefetch it here as first thing to hide some of the
  	 * latency.
  	 *
  	 * Attempt to prefetch the pieces we likely need later.
  	 */
  	prefetch(&bdev->bd_disk->part_tbl);
  	prefetch(bdev->bd_queue);
  	prefetch((char *)bdev->bd_queue + SMP_CACHE_BYTES);

  	return do_blockdev_direct_IO(iocb, inode, bdev, iter, get_block,

  				     end_io, submit_io, flags);

  }

  EXPORT_SYMBOL(__blockdev_direct_IO);

  
  static __init int dio_init(void)
  {
  	dio_cache = KMEM_CACHE(dio, SLAB_PANIC);
  	return 0;
  }
  module_init(dio_init)