/*
   * 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 <asm/atomic.h>
  
  /*
   * How many user pages to map in one call to get_user_pages().  This determines
   * the size of a structure on the stack.
   */
  #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.
   *
   * lock_type is DIO_LOCKING for regular files on direct-IO-naive filesystems.
   * This determines whether we need to do the fancy locking which prevents
   * direct-IO from being able to read uninitialised disk blocks.  If its zero

   * (blockdev) this locking is not done, and if it is DIO_OWN_LOCKING i_mutex is

   * not held for the entire direct write (taken briefly, initially, during a
   * direct read though, but its never held for the duration of a direct-IO).
   */
  
  struct dio {
  	/* BIO submission state */
  	struct bio *bio;		/* bio under assembly */
  	struct inode *inode;
  	int rw;

  	loff_t i_size;			/* i_size when submitted */

  	int lock_type;			/* doesn't change */
  	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 */
  	size_t	size;			/* total request size (doesn't change)*/
  	sector_t block_in_file;		/* Current offset into the underlying
  					   file in dio_block units. */
  	unsigned blocks_available;	/* At block_in_file.  changes */
  	sector_t final_block_in_request;/* doesn't change */
  	unsigned first_block_in_page;	/* doesn't change, Used only once */
  	int boundary;			/* prev block is at a boundary */
  	int reap_counter;		/* rate limit reaping */

  	get_block_t *get_block;		/* block mapping function */

  	dio_iodone_t *end_io;		/* IO completion function */
  	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 */

  	struct buffer_head map_bh;	/* last get_block() result */

  
  	/*
  	 * 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 */
  
  	/*
  	 * Page fetching state. These variables belong to dio_refill_pages().
  	 */
  	int curr_page;			/* changes */
  	int total_pages;		/* doesn't change */
  	unsigned long curr_user_address;/* changes */
  
  	/*
  	 * Page queue.  These variables belong to dio_refill_pages() and
  	 * dio_get_page().
  	 */
  	struct page *pages[DIO_PAGES];	/* page buffer */
  	unsigned head;			/* next page to process */
  	unsigned tail;			/* last valid page + 1 */
  	int page_errors;		/* errno from get_user_pages() */
  
  	/* BIO completion state */
  	spinlock_t bio_lock;		/* protects BIO fields below */

  	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 */
  	int is_async;			/* is IO async ? */

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

  	ssize_t result;                 /* IO result */
  };
  
  /*
   * How many pages are in the queue?
   */
  static inline unsigned dio_pages_present(struct dio *dio)
  {
  	return dio->tail - dio->head;
  }
  
  /*
   * Go grab and pin some userspace pages.   Typically we'll get 64 at a time.
   */
  static int dio_refill_pages(struct dio *dio)
  {
  	int ret;
  	int nr_pages;
  
  	nr_pages = min(dio->total_pages - dio->curr_page, DIO_PAGES);

  	ret = get_user_pages_fast(

  		dio->curr_user_address,		/* Where from? */
  		nr_pages,			/* How many pages? */
  		dio->rw == READ,		/* Write to memory? */

  		&dio->pages[0]);		/* Put results here */

  	if (ret < 0 && dio->blocks_available && (dio->rw & 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;

  		page_cache_get(page);
  		dio->pages[0] = page;

  		dio->head = 0;
  		dio->tail = 1;
  		ret = 0;
  		goto out;
  	}
  
  	if (ret >= 0) {
  		dio->curr_user_address += ret * PAGE_SIZE;
  		dio->curr_page += ret;
  		dio->head = 0;
  		dio->tail = ret;
  		ret = 0;
  	}
  out:
  	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 struct page *dio_get_page(struct dio *dio)
  {
  	if (dio_pages_present(dio) == 0) {
  		int ret;
  
  		ret = dio_refill_pages(dio);
  		if (ret)
  			return ERR_PTR(ret);
  		BUG_ON(dio_pages_present(dio) == 0);
  	}
  	return dio->pages[dio->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 releases locks as dictated by the locking type, 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 int dio_complete(struct dio *dio, loff_t offset, int ret)

  {

  	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->rw == READ) && ((offset + transferred) > dio->i_size))
  			transferred = dio->i_size - offset;
  	}

  	if (dio->end_io && dio->result)

  		dio->end_io(dio->iocb, offset, transferred,
  			    dio->map_bh.b_private);

  	if (dio->lock_type == DIO_LOCKING)

  		/* lockdep: non-owner release */
  		up_read_non_owner(&dio->inode->i_alloc_sem);

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

  }

  static int dio_bio_complete(struct dio *dio, struct bio *bio);
  /*
   * Asynchronous IO callback. 
   */

  static void dio_bio_end_aio(struct bio *bio, int error)

  {
  	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) {
  		int ret = dio_complete(dio, dio->iocb->ki_pos, 0);
  		aio_complete(dio->iocb, ret, 0);
  		kfree(dio);
  	}

  }
  
  /*
   * 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, int error)

  {
  	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);

  }
  
  static int
  dio_bio_alloc(struct dio *dio, struct block_device *bdev,
  		sector_t first_sector, int nr_vecs)
  {
  	struct bio *bio;
  
  	bio = bio_alloc(GFP_KERNEL, nr_vecs);

  
  	bio->bi_bdev = bdev;
  	bio->bi_sector = first_sector;
  	if (dio->is_async)
  		bio->bi_end_io = dio_bio_end_aio;
  	else
  		bio->bi_end_io = dio_bio_end_io;
  
  	dio->bio = bio;
  	return 0;
  }
  
  /*
   * 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 void dio_bio_submit(struct dio *dio)
  {
  	struct bio *bio = dio->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->rw == READ)
  		bio_set_pages_dirty(bio);

  	submit_bio(dio->rw, bio);
  
  	dio->bio = NULL;
  	dio->boundary = 0;
  }
  
  /*
   * Release any resources in case of a failure
   */
  static void dio_cleanup(struct dio *dio)
  {
  	while (dio_pages_present(dio))
  		page_cache_release(dio_get_page(dio));
  }
  
  /*

   * 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);
  		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)
  {
  	const int uptodate = test_bit(BIO_UPTODATE, &bio->bi_flags);
  	struct bio_vec *bvec = bio->bi_io_vec;
  	int page_no;
  
  	if (!uptodate)

  		dio->io_error = -EIO;

  
  	if (dio->is_async && dio->rw == READ) {
  		bio_check_pages_dirty(bio);	/* transfers ownership */
  	} else {
  		for (page_no = 0; page_no < bio->bi_vcnt; page_no++) {
  			struct page *page = bvec[page_no].bv_page;
  
  			if (dio->rw == READ && !PageCompound(page))
  				set_page_dirty_lock(page);
  			page_cache_release(page);
  		}
  		bio_put(bio);
  	}

  	return uptodate ? 0 : -EIO;
  }
  
  /*

   * 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 int dio_bio_reap(struct dio *dio)
  {
  	int ret = 0;
  
  	if (dio->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;
  		}
  		dio->reap_counter = 0;
  	}
  	return ret;
  }
  
  /*
   * Call into the fs to map some more disk blocks.  We record the current number
   * of available blocks at dio->blocks_available.  These are in units of the
   * fs blocksize, (1 << inode->i_blkbits).
   *
   * 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)
  {
  	int ret;
  	struct buffer_head *map_bh = &dio->map_bh;
  	sector_t fs_startblk;	/* Into file, in filesystem-sized blocks */
  	unsigned long fs_count;	/* Number of filesystem-sized blocks */
  	unsigned long dio_count;/* Number of dio_block-sized blocks */
  	unsigned long blkmask;
  	int create;
  
  	/*
  	 * 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(dio->block_in_file >= dio->final_block_in_request);
  		fs_startblk = dio->block_in_file >> dio->blkfactor;
  		dio_count = dio->final_block_in_request - dio->block_in_file;
  		fs_count = dio_count >> dio->blkfactor;
  		blkmask = (1 << dio->blkfactor) - 1;
  		if (dio_count & blkmask)	
  			fs_count++;

  		map_bh->b_state = 0;
  		map_bh->b_size = fs_count << dio->inode->i_blkbits;

  		create = dio->rw & WRITE;

  		if (dio->lock_type == DIO_LOCKING) {
  			if (dio->block_in_file < (i_size_read(dio->inode) >>
  							dio->blkbits))
  				create = 0;
  		} else if (dio->lock_type == DIO_NO_LOCKING) {
  			create = 0;
  		}

  		/*
  		 * For writes inside i_size we forbid block creations: only
  		 * overwrites are permitted.  We fall back to buffered writes
  		 * at a higher level for inside-i_size block-instantiating
  		 * writes.
  		 */

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

  						map_bh, create);
  	}
  	return ret;
  }
  
  /*
   * There is no bio.  Make one now.
   */
  static int dio_new_bio(struct dio *dio, sector_t start_sector)
  {
  	sector_t sector;
  	int ret, nr_pages;
  
  	ret = dio_bio_reap(dio);
  	if (ret)
  		goto out;
  	sector = start_sector << (dio->blkbits - 9);
  	nr_pages = min(dio->pages_in_io, bio_get_nr_vecs(dio->map_bh.b_bdev));
  	BUG_ON(nr_pages <= 0);
  	ret = dio_bio_alloc(dio, dio->map_bh.b_bdev, sector, nr_pages);
  	dio->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 int dio_bio_add_page(struct dio *dio)
  {
  	int ret;
  
  	ret = bio_add_page(dio->bio, dio->cur_page,
  			dio->cur_page_len, dio->cur_page_offset);
  	if (ret == dio->cur_page_len) {
  		/*
  		 * Decrement count only, if we are done with this page
  		 */
  		if ((dio->cur_page_len + dio->cur_page_offset) == PAGE_SIZE)
  			dio->pages_in_io--;
  		page_cache_get(dio->cur_page);
  		dio->final_block_in_bio = dio->cur_page_block +
  			(dio->cur_page_len >> dio->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 int dio_send_cur_page(struct dio *dio)
  {
  	int ret = 0;
  
  	if (dio->bio) {
  		/*
  		 * See whether this new request is contiguous with the old
  		 */
  		if (dio->final_block_in_bio != dio->cur_page_block)
  			dio_bio_submit(dio);
  		/*
  		 * Submit now if the underlying fs is about to perform a
  		 * metadata read
  		 */
  		if (dio->boundary)
  			dio_bio_submit(dio);
  	}
  
  	if (dio->bio == NULL) {
  		ret = dio_new_bio(dio, dio->cur_page_block);
  		if (ret)
  			goto out;
  	}
  
  	if (dio_bio_add_page(dio) != 0) {
  		dio_bio_submit(dio);
  		ret = dio_new_bio(dio, dio->cur_page_block);
  		if (ret == 0) {
  			ret = dio_bio_add_page(dio);
  			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 int
  submit_page_section(struct dio *dio, struct page *page,
  		unsigned offset, unsigned len, sector_t blocknr)
  {
  	int ret = 0;

  	if (dio->rw & 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 (	(dio->cur_page == page) &&
  		(dio->cur_page_offset + dio->cur_page_len == offset) &&
  		(dio->cur_page_block +
  			(dio->cur_page_len >> dio->blkbits) == blocknr)) {
  		dio->cur_page_len += len;
  
  		/*
  		 * If dio->boundary then we want to schedule the IO now to
  		 * avoid metadata seeks.
  		 */
  		if (dio->boundary) {
  			ret = dio_send_cur_page(dio);
  			page_cache_release(dio->cur_page);
  			dio->cur_page = NULL;
  		}
  		goto out;
  	}
  
  	/*
  	 * If there's a deferred page already there then send it.
  	 */
  	if (dio->cur_page) {
  		ret = dio_send_cur_page(dio);
  		page_cache_release(dio->cur_page);
  		dio->cur_page = NULL;
  		if (ret)
  			goto out;
  	}
  
  	page_cache_get(page);		/* It is in dio */
  	dio->cur_page = page;
  	dio->cur_page_offset = offset;
  	dio->cur_page_len = len;
  	dio->cur_page_block = blocknr;
  out:
  	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)
  {
  	unsigned i;
  	unsigned nblocks;
  
  	nblocks = dio->map_bh.b_size >> dio->inode->i_blkbits;
  
  	for (i = 0; i < nblocks; i++) {
  		unmap_underlying_metadata(dio->map_bh.b_bdev,
  					dio->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 void dio_zero_block(struct dio *dio, int end)
  {
  	unsigned dio_blocks_per_fs_block;
  	unsigned this_chunk_blocks;	/* In dio_blocks */
  	unsigned this_chunk_bytes;
  	struct page *page;
  
  	dio->start_zero_done = 1;
  	if (!dio->blkfactor || !buffer_new(&dio->map_bh))
  		return;
  
  	dio_blocks_per_fs_block = 1 << dio->blkfactor;
  	this_chunk_blocks = dio->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 << dio->blkbits;

  	page = ZERO_PAGE(0);

  	if (submit_page_section(dio, page, 0, this_chunk_bytes, 
  				dio->next_block_for_io))
  		return;
  
  	dio->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)
  {
  	const unsigned blkbits = dio->blkbits;
  	const unsigned blocks_per_page = PAGE_SIZE >> blkbits;
  	struct page *page;
  	unsigned block_in_page;
  	struct buffer_head *map_bh = &dio->map_bh;
  	int ret = 0;
  
  	/* The I/O can start at any block offset within the first page */
  	block_in_page = dio->first_block_in_page;
  
  	while (dio->block_in_file < dio->final_block_in_request) {
  		page = dio_get_page(dio);
  		if (IS_ERR(page)) {
  			ret = PTR_ERR(page);
  			goto out;
  		}
  
  		while (block_in_page < blocks_per_page) {
  			unsigned offset_in_page = block_in_page << blkbits;
  			unsigned this_chunk_bytes;	/* # of bytes mapped */
  			unsigned this_chunk_blocks;	/* # of blocks */
  			unsigned u;
  
  			if (dio->blocks_available == 0) {
  				/*
  				 * Need to go and map some more disk
  				 */
  				unsigned long blkmask;
  				unsigned long dio_remainder;
  
  				ret = get_more_blocks(dio);
  				if (ret) {
  					page_cache_release(page);
  					goto out;
  				}
  				if (!buffer_mapped(map_bh))
  					goto do_holes;
  
  				dio->blocks_available =
  						map_bh->b_size >> dio->blkbits;
  				dio->next_block_for_io =
  					map_bh->b_blocknr << dio->blkfactor;
  				if (buffer_new(map_bh))
  					clean_blockdev_aliases(dio);
  
  				if (!dio->blkfactor)
  					goto do_holes;
  
  				blkmask = (1 << dio->blkfactor) - 1;
  				dio_remainder = (dio->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))
  					dio->next_block_for_io += dio_remainder;
  				dio->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->rw & WRITE) {

  					page_cache_release(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 (dio->block_in_file >=

  						i_size_aligned >> blkbits) {

  					/* We hit eof */
  					page_cache_release(page);
  					goto out;
  				}

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

  				dio->block_in_file++;
  				block_in_page++;
  				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(dio->blkfactor && !dio->start_zero_done))
  				dio_zero_block(dio, 0);
  
  			/*
  			 * Work out, in this_chunk_blocks, how much disk we
  			 * can add to this page
  			 */
  			this_chunk_blocks = dio->blocks_available;
  			u = (PAGE_SIZE - offset_in_page) >> blkbits;
  			if (this_chunk_blocks > u)
  				this_chunk_blocks = u;
  			u = dio->final_block_in_request - dio->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);
  
  			dio->boundary = buffer_boundary(map_bh);
  			ret = submit_page_section(dio, page, offset_in_page,
  				this_chunk_bytes, dio->next_block_for_io);
  			if (ret) {
  				page_cache_release(page);
  				goto out;
  			}
  			dio->next_block_for_io += this_chunk_blocks;
  
  			dio->block_in_file += this_chunk_blocks;
  			block_in_page += this_chunk_blocks;
  			dio->blocks_available -= this_chunk_blocks;
  next_block:

  			BUG_ON(dio->block_in_file > dio->final_block_in_request);

  			if (dio->block_in_file == dio->final_block_in_request)
  				break;
  		}
  
  		/* Drop the ref which was taken in get_user_pages() */
  		page_cache_release(page);
  		block_in_page = 0;
  	}
  out:
  	return ret;
  }
  
  /*

   * Releases both i_mutex and i_alloc_sem

   */
  static ssize_t
  direct_io_worker(int rw, struct kiocb *iocb, struct inode *inode, 
  	const struct iovec *iov, loff_t offset, unsigned long nr_segs, 

  	unsigned blkbits, get_block_t get_block, dio_iodone_t end_io,

  	struct dio *dio)
  {
  	unsigned long user_addr; 

  	unsigned long flags;

  	int seg;
  	ssize_t ret = 0;
  	ssize_t ret2;
  	size_t bytes;

  	dio->inode = inode;
  	dio->rw = rw;
  	dio->blkbits = blkbits;
  	dio->blkfactor = inode->i_blkbits - blkbits;

  	dio->block_in_file = offset >> blkbits;

  	dio->get_block = get_block;

  	dio->end_io = end_io;

  	dio->final_block_in_bio = -1;
  	dio->next_block_for_io = -1;

  	dio->iocb = iocb;

  	dio->i_size = i_size_read(inode);

  	spin_lock_init(&dio->bio_lock);

  	dio->refcount = 1;

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

  
  	for (seg = 0; seg < nr_segs; seg++) {
  		user_addr = (unsigned long)iov[seg].iov_base;
  		dio->pages_in_io +=
  			((user_addr+iov[seg].iov_len +PAGE_SIZE-1)/PAGE_SIZE
  				- user_addr/PAGE_SIZE);
  	}
  
  	for (seg = 0; seg < nr_segs; seg++) {
  		user_addr = (unsigned long)iov[seg].iov_base;
  		dio->size += bytes = iov[seg].iov_len;
  
  		/* Index into the first page of the first block */
  		dio->first_block_in_page = (user_addr & ~PAGE_MASK) >> blkbits;
  		dio->final_block_in_request = dio->block_in_file +
  						(bytes >> blkbits);
  		/* Page fetching state */
  		dio->head = 0;
  		dio->tail = 0;
  		dio->curr_page = 0;
  
  		dio->total_pages = 0;
  		if (user_addr & (PAGE_SIZE-1)) {
  			dio->total_pages++;
  			bytes -= PAGE_SIZE - (user_addr & (PAGE_SIZE - 1));
  		}
  		dio->total_pages += (bytes + PAGE_SIZE - 1) / PAGE_SIZE;
  		dio->curr_user_address = user_addr;
  	
  		ret = do_direct_IO(dio);
  
  		dio->result += iov[seg].iov_len -
  			((dio->final_block_in_request - dio->block_in_file) <<
  					blkbits);
  
  		if (ret) {
  			dio_cleanup(dio);
  			break;
  		}
  	} /* end iovec loop */

  	if (ret == -ENOTBLK && (rw & WRITE)) {

  		/*
  		 * The remaining part of the request will be
  		 * be handled by buffered I/O when we return
  		 */
  		ret = 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, 1);
  
  	if (dio->cur_page) {
  		ret2 = dio_send_cur_page(dio);
  		if (ret == 0)
  			ret = ret2;
  		page_cache_release(dio->cur_page);
  		dio->cur_page = NULL;
  	}
  	if (dio->bio)
  		dio_bio_submit(dio);

  	/* All IO is now issued, send it on its way */
  	blk_run_address_space(inode->i_mapping);

  	/*
  	 * 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);
  
  	/*
  	 * 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 ((rw == READ) && (dio->lock_type == DIO_LOCKING))

  		mutex_unlock(&dio->inode->i_mutex);

  
  	/*

  	 * 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(ret == -EIOCBQUEUED);
  	if (dio->is_async && ret == 0 && dio->result &&
  	    ((rw & READ) || (dio->result == dio->size)))
  		ret = -EIOCBQUEUED;

  	if (ret != -EIOCBQUEUED)

  		dio_await_completion(dio);

  	/*
  	 * 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 aio_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);

  	if (ret2 == 0) {

  		ret = dio_complete(dio, offset, ret);

  		kfree(dio);
  	} else
  		BUG_ON(ret != -EIOCBQUEUED);

  	return ret;
  }
  
  /*
   * This is a library function for use by filesystem drivers.
   * The locking rules are governed by the dio_lock_type parameter.
   *
   * DIO_NO_LOCKING (no locking, for raw block device access)

   * For writes, i_mutex is not held on entry; it is never taken.

   *
   * DIO_LOCKING (simple locking for regular files)

   * For writes we are called under i_mutex and return with i_mutex held, even
   * though it is internally dropped.

   * For reads, i_mutex is not held on entry, but it is taken and dropped before

   * returning.
   *
   * DIO_OWN_LOCKING (filesystem provides synchronisation and handling of
   *	uninitialised data, allowing parallel direct readers and writers)

   * For writes we are called without i_mutex, return without it, never touch it.

   * For reads we are called under i_mutex and return with i_mutex held, even
   * though it may be internally dropped.

   *
   * Additional i_alloc_sem locking requirements described inline below.
   */
  ssize_t
  __blockdev_direct_IO(int rw, struct kiocb *iocb, struct inode *inode,
  	struct block_device *bdev, const struct iovec *iov, loff_t offset, 

  	unsigned long nr_segs, get_block_t get_block, dio_iodone_t end_io,

  	int dio_lock_type)
  {
  	int seg;
  	size_t size;
  	unsigned long addr;
  	unsigned blkbits = inode->i_blkbits;
  	unsigned bdev_blkbits = 0;
  	unsigned blocksize_mask = (1 << blkbits) - 1;
  	ssize_t retval = -EINVAL;
  	loff_t end = offset;
  	struct dio *dio;

  	int release_i_mutex = 0;
  	int acquire_i_mutex = 0;

  
  	if (rw & WRITE)

  		rw = WRITE_ODIRECT;

  
  	if (bdev)
  		bdev_blkbits = blksize_bits(bdev_hardsect_size(bdev));
  
  	if (offset & blocksize_mask) {
  		if (bdev)
  			 blkbits = bdev_blkbits;
  		blocksize_mask = (1 << blkbits) - 1;
  		if (offset & blocksize_mask)
  			goto out;
  	}
  
  	/* Check the memory alignment.  Blocks cannot straddle pages */
  	for (seg = 0; seg < nr_segs; seg++) {
  		addr = (unsigned long)iov[seg].iov_base;
  		size = iov[seg].iov_len;
  		end += size;
  		if ((addr & blocksize_mask) || (size & blocksize_mask))  {
  			if (bdev)
  				 blkbits = bdev_blkbits;
  			blocksize_mask = (1 << blkbits) - 1;
  			if ((addr & blocksize_mask) || (size & blocksize_mask))  
  				goto out;
  		}
  	}

  	dio = kzalloc(sizeof(*dio), GFP_KERNEL);

  	retval = -ENOMEM;
  	if (!dio)
  		goto out;
  
  	/*
  	 * For block device access DIO_NO_LOCKING is used,
  	 *	neither readers nor writers do any locking at all
  	 * For regular files using DIO_LOCKING,

  	 *	readers need to grab i_mutex and i_alloc_sem
  	 *	writers need to grab i_alloc_sem only (i_mutex is already held)

  	 * For regular files using DIO_OWN_LOCKING,
  	 *	neither readers nor writers take any locks here

  	 */
  	dio->lock_type = dio_lock_type;
  	if (dio_lock_type != DIO_NO_LOCKING) {
  		/* watch out for a 0 len io from a tricksy fs */
  		if (rw == READ && end > offset) {
  			struct address_space *mapping;
  
  			mapping = iocb->ki_filp->f_mapping;
  			if (dio_lock_type != DIO_OWN_LOCKING) {

  				mutex_lock(&inode->i_mutex);

  				release_i_mutex = 1;

  			}
  
  			retval = filemap_write_and_wait_range(mapping, offset,
  							      end - 1);
  			if (retval) {
  				kfree(dio);
  				goto out;
  			}
  
  			if (dio_lock_type == DIO_OWN_LOCKING) {

  				mutex_unlock(&inode->i_mutex);

  				acquire_i_mutex = 1;

  			}
  		}
  
  		if (dio_lock_type == DIO_LOCKING)

  			/* lockdep: not the owner will release it */
  			down_read_non_owner(&inode->i_alloc_sem);

  	}
  
  	/*
  	 * For file extending writes updating i_size before data
  	 * writeouts complete can expose uninitialized blocks. So
  	 * even for AIO, we need to wait for i/o to complete before
  	 * returning in this case.
  	 */

  	dio->is_async = !is_sync_kiocb(iocb) && !((rw & WRITE) &&

  		(end > i_size_read(inode)));
  
  	retval = direct_io_worker(rw, iocb, inode, iov, offset,

  				nr_segs, blkbits, get_block, end_io, dio);

  	/*
  	 * In case of error extending write may have instantiated a few
  	 * blocks outside i_size. Trim these off again for DIO_LOCKING.
  	 * NOTE: DIO_NO_LOCK/DIO_OWN_LOCK callers have to handle this by
  	 * it's own meaner.
  	 */
  	if (unlikely(retval < 0 && (rw & WRITE))) {
  		loff_t isize = i_size_read(inode);
  
  		if (end > isize && dio_lock_type == DIO_LOCKING)
  			vmtruncate(inode, isize);
  	}

  	if (rw == READ && dio_lock_type == DIO_LOCKING)

  		release_i_mutex = 0;

  
  out:

  	if (release_i_mutex)

  		mutex_unlock(&inode->i_mutex);

  	else if (acquire_i_mutex)
  		mutex_lock(&inode->i_mutex);

  	return retval;
  }
  EXPORT_SYMBOL(__blockdev_direct_IO);