Message-ID: <87tykavom7.fsf@dmon-lap.sw.ru>
Date:	Mon, 25 Oct 2010 09:16:16 +0400
From:	Dmitry <dmonakhov@...nvz.org>
To:	Theodore Ts'o <tytso@....edu>, linux-ext4@...r.kernel.org
Cc:	akpm@...ux-foundation.org, Theodore Ts'o <tytso@....edu>
Subject: Re: [PATCH -v2 6/6] ext4: use bio layer instead of buffer layer in mpage_da_submit_io

On Sat, 23 Oct 2010 16:40:20 -0400, Theodore Ts'o <tytso@....edu> wrote:
> Call the block I/O layer directly instad of going through the buffer
> layer.  This should give us much better performance and scalability,
> as well as lowering our CPU utilization when doing buffered writeback.
> 
> Signed-off-by: "Theodore Ts'o" <tytso@....edu>
> ---
>  fs/ext4/Makefile  |    2 +-
>  fs/ext4/ext4.h    |   36 +++++-
>  fs/ext4/extents.c |    4 +-
>  fs/ext4/inode.c   |  118 ++-------------
>  fs/ext4/page-io.c |  426 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  fs/ext4/super.c   |    8 +-
>  6 files changed, 485 insertions(+), 109 deletions(-)
>  create mode 100644 fs/ext4/page-io.c
> 
> diff --git a/fs/ext4/Makefile b/fs/ext4/Makefile
> index 8867b2a..c947e36 100644
> --- a/fs/ext4/Makefile
> +++ b/fs/ext4/Makefile
> @@ -4,7 +4,7 @@
>  
>  obj-$(CONFIG_EXT4_FS) += ext4.o
>  
> -ext4-y	:= balloc.o bitmap.o dir.o file.o fsync.o ialloc.o inode.o \
> +ext4-y	:= balloc.o bitmap.o dir.o file.o fsync.o ialloc.o inode.o page-io.o \
>  		ioctl.o namei.o super.o symlink.o hash.o resize.o extents.o \
>  		ext4_jbd2.o migrate.o mballoc.o block_validity.o move_extent.o
>  
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 2283369..3d1abd0 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -168,7 +168,20 @@ struct mpage_da_data {
>  	int pages_written;
>  	int retval;
>  };
> -#define	EXT4_IO_UNWRITTEN	0x1
> +
> +/*
> + * Flags for ext4_io_end->flags
> + */
> +#define	EXT4_IO_END_UNWRITTEN	0x0001
> +#define EXT4_IO_END_ERROR	0x0002
> +
> +struct ext4_io_page {
> +	struct page	*p_page;
> +	int		p_count;
> +};
> +
> +#define MAX_IO_PAGES 128
> +
>  typedef struct ext4_io_end {
>  	struct list_head	list;		/* per-file finished IO list */
>  	struct inode		*inode;		/* file being written to */
> @@ -179,8 +192,18 @@ typedef struct ext4_io_end {
>  	struct work_struct	work;		/* data work queue */
>  	struct kiocb		*iocb;		/* iocb struct for AIO */
>  	int			result;		/* error value for AIO */
> +	int			num_io_pages;
> +	struct ext4_io_page	*pages[MAX_IO_PAGES];
>  } ext4_io_end_t;
>  
> +struct ext4_io_submit {
> +	int			io_op;
> +	struct bio		*io_bio;
> +	ext4_io_end_t		*io_end;
> +	struct ext4_io_page	*io_page;
> +	sector_t		io_next_block;
> +};
> +
>  /*
>   * Special inodes numbers
>   */
> @@ -2044,6 +2067,17 @@ extern int ext4_move_extents(struct file *o_filp, struct file *d_filp,
>  			     __u64 start_orig, __u64 start_donor,
>  			     __u64 len, __u64 *moved_len);
>  
> +/* page_io.c */
> +extern int __init init_ext4_pageio(void);
> +extern void exit_ext4_pageio(void);
> +extern void ext4_free_io_end(ext4_io_end_t *io);
> +extern ext4_io_end_t *ext4_init_io_end(struct inode *inode, gfp_t flags);
> +extern int ext4_end_io_nolock(ext4_io_end_t *io);
> +extern void ext4_io_submit(struct ext4_io_submit *io);
> +extern int ext4_bio_write_page(struct ext4_io_submit *io,
> +			       struct page *page,
> +			       int len,
> +			       struct writeback_control *wbc);
>  
>  /* BH_Uninit flag: blocks are allocated but uninitialized on disk */
>  enum ext4_state_bits {
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index a0e6230..a1e20c8 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -3202,7 +3202,7 @@ ext4_ext_handle_uninitialized_extents(handle_t *handle, struct inode *inode,
>  		 * completed
>  		 */
>  		if (io)
> -			io->flag = EXT4_IO_UNWRITTEN;
> +			io->flag = EXT4_IO_END_UNWRITTEN;
>  		else
>  			ext4_set_inode_state(inode, EXT4_STATE_DIO_UNWRITTEN);
>  		if (ext4_should_dioread_nolock(inode))
> @@ -3494,7 +3494,7 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
>  		 */
>  		if ((flags & EXT4_GET_BLOCKS_PRE_IO)) {
>  			if (io)
> -				io->flag = EXT4_IO_UNWRITTEN;
> +				io->flag = EXT4_IO_END_UNWRITTEN;
>  			else
>  				ext4_set_inode_state(inode,
>  						     EXT4_STATE_DIO_UNWRITTEN);
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index c65d647..58604fe 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -2016,8 +2016,10 @@ static int mpage_da_submit_io(struct mpage_da_data *mpd,
>  	struct buffer_head *bh, *page_bufs = NULL;
>  	int journal_data = ext4_should_journal_data(inode);
>  	sector_t pblock = 0, cur_logical = 0;
> +	struct ext4_io_submit io_submit;
>  
>  	BUG_ON(mpd->next_page <= mpd->first_page);
> +	memset(&io_submit, 0, sizeof(io_submit));
>  	/*
>  	 * We need to start from the first_page to the next_page - 1
>  	 * to make sure we also write the mapped dirty buffer_heads.
> @@ -2109,16 +2111,16 @@ static int mpage_da_submit_io(struct mpage_da_data *mpd,
>  				/* mark the buffer_heads as dirty & uptodate */
>  				block_commit_write(page, 0, len);
>  
> -			if (journal_data && PageChecked(page))
> +			/*
> +			 * Delalloc doesn't support data journalling,
> +			 * but eventually maybe we'll lift this
> +			 * restriction.
> +			 */
> +			if (unlikely(journal_data && PageChecked(page)))
>  				err = __ext4_journalled_writepage(page, len);
> -			else if (buffer_uninit(page_bufs)) {
> -				ext4_set_bh_endio(page_bufs, inode);
> -				err = block_write_full_page_endio(page,
> -					noalloc_get_block_write,
> -					mpd->wbc, ext4_end_io_buffer_write);
> -			} else
> -				err = block_write_full_page(page,
> -					    noalloc_get_block_write, mpd->wbc);
> +			else
> +				err = ext4_bio_write_page(&io_submit, page,
> +							  len, mpd->wbc);
>  
>  			if (!err)
>  				mpd->pages_written++;
> @@ -2131,6 +2133,7 @@ static int mpage_da_submit_io(struct mpage_da_data *mpd,
>  		}
>  		pagevec_release(&pvec);
>  	}
> +	ext4_io_submit(&io_submit);
>  	return ret;
>  }
>  
> @@ -3426,15 +3429,6 @@ ext4_readpages(struct file *file, struct address_space *mapping,
>  	return mpage_readpages(mapping, pages, nr_pages, ext4_get_block);
>  }
>  
> -static void ext4_free_io_end(ext4_io_end_t *io)
> -{
> -	BUG_ON(!io);
> -	if (io->page)
> -		put_page(io->page);
> -	iput(io->inode);
> -	kfree(io);
> -}
> -
>  static void ext4_invalidatepage_free_endio(struct page *page, unsigned long offset)
>  {
>  	struct buffer_head *head, *bh;
> @@ -3640,68 +3634,6 @@ static void dump_completed_IO(struct inode * inode)
>  }
>  
>  /*
> - * check a range of space and convert unwritten extents to written.
> - */
> -static int ext4_end_io_nolock(ext4_io_end_t *io)
> -{
> -	struct inode *inode = io->inode;
> -	loff_t offset = io->offset;
> -	ssize_t size = io->size;
> -	int ret = 0;
> -
> -	ext4_debug("ext4_end_io_nolock: io 0x%p from inode %lu,list->next 0x%p,"
> -		   "list->prev 0x%p\n",
> -	           io, inode->i_ino, io->list.next, io->list.prev);
> -
> -	if (list_empty(&io->list))
> -		return ret;
> -
> -	if (io->flag != EXT4_IO_UNWRITTEN)
> -		return ret;
> -
> -	ret = ext4_convert_unwritten_extents(inode, offset, size);
> -	if (ret < 0) {
> -		printk(KERN_EMERG "%s: failed to convert unwritten"
> -			"extents to written extents, error is %d"
> -			" io is still on inode %lu aio dio list\n",
> -                       __func__, ret, inode->i_ino);
> -		return ret;
> -	}
> -
> -	if (io->iocb)
> -		aio_complete(io->iocb, io->result, 0);
> -	/* clear the DIO AIO unwritten flag */
> -	io->flag = 0;
> -	return ret;
> -}
> -
> -/*
> - * work on completed aio dio IO, to convert unwritten extents to extents
> - */
> -static void ext4_end_io_work(struct work_struct *work)
> -{
> -	ext4_io_end_t		*io = container_of(work, ext4_io_end_t, work);
> -	struct inode		*inode = io->inode;
> -	struct ext4_inode_info	*ei = EXT4_I(inode);
> -	unsigned long		flags;
> -	int			ret;
> -
> -	mutex_lock(&inode->i_mutex);
> -	ret = ext4_end_io_nolock(io);
> -	if (ret < 0) {
> -		mutex_unlock(&inode->i_mutex);
> -		return;
> -	}
> -
> -	spin_lock_irqsave(&ei->i_completed_io_lock, flags);
> -	if (!list_empty(&io->list))
> -		list_del_init(&io->list);
> -	spin_unlock_irqrestore(&ei->i_completed_io_lock, flags);
> -	mutex_unlock(&inode->i_mutex);
> -	ext4_free_io_end(io);
> -}
> -
> -/*
>   * This function is called from ext4_sync_file().
>   *
>   * When IO is completed, the work to convert unwritten extents to
> @@ -3756,28 +3688,6 @@ int flush_completed_IO(struct inode *inode)
>  	return (ret2 < 0) ? ret2 : 0;
>  }
>  
> -static ext4_io_end_t *ext4_init_io_end (struct inode *inode, gfp_t flags)
> -{
> -	ext4_io_end_t *io = NULL;
> -
> -	io = kmalloc(sizeof(*io), flags);
> -
> -	if (io) {
> -		igrab(inode);
> -		io->inode = inode;
> -		io->flag = 0;
> -		io->offset = 0;
> -		io->size = 0;
> -		io->page = NULL;
> -		io->iocb = NULL;
> -		io->result = 0;
> -		INIT_WORK(&io->work, ext4_end_io_work);
> -		INIT_LIST_HEAD(&io->list);
> -	}
> -
> -	return io;
> -}
> -
>  static void ext4_end_io_dio(struct kiocb *iocb, loff_t offset,
>  			    ssize_t size, void *private, int ret,
>  			    bool is_async)
> @@ -3797,7 +3707,7 @@ static void ext4_end_io_dio(struct kiocb *iocb, loff_t offset,
>  		  size);
>  
>  	/* if not aio dio with unwritten extents, just free io and return */
> -	if (io_end->flag != EXT4_IO_UNWRITTEN){
> +	if (!(io_end->flag & EXT4_IO_END_UNWRITTEN)) {
>  		ext4_free_io_end(io_end);
>  		iocb->private = NULL;
>  out:
> @@ -3842,7 +3752,7 @@ static void ext4_end_io_buffer_write(struct buffer_head *bh, int uptodate)
>  		goto out;
>  	}
>  
> -	io_end->flag = EXT4_IO_UNWRITTEN;
> +	io_end->flag = EXT4_IO_END_UNWRITTEN;
>  	inode = io_end->inode;
>  
>  	/* Add the io_end to per-inode completed io list*/
> diff --git a/fs/ext4/page-io.c b/fs/ext4/page-io.c
> new file mode 100644
> index 0000000..ec92e38
> --- /dev/null
> +++ b/fs/ext4/page-io.c
> @@ -0,0 +1,426 @@
> +/*
> + * linux/fs/ext4/page-io.c
> + *
> + * This contains the new page_io functions for ext4
> + *
> + * Written by Theodore Ts'o, 2010.
> + */
> +
> +#include <linux/module.h>
> +#include <linux/fs.h>
> +#include <linux/time.h>
> +#include <linux/jbd2.h>
> +#include <linux/highuid.h>
> +#include <linux/pagemap.h>
> +#include <linux/quotaops.h>
> +#include <linux/string.h>
> +#include <linux/buffer_head.h>
> +#include <linux/writeback.h>
> +#include <linux/pagevec.h>
> +#include <linux/mpage.h>
> +#include <linux/namei.h>
> +#include <linux/uio.h>
> +#include <linux/bio.h>
> +#include <linux/workqueue.h>
> +#include <linux/kernel.h>
> +#include <linux/slab.h>
> +
> +#include "ext4_jbd2.h"
> +#include "xattr.h"
> +#include "acl.h"
> +#include "ext4_extents.h"
> +
> +static struct kmem_cache *io_page_cachep, *io_end_cachep;
> +
> +int __init init_ext4_pageio(void)
> +{
> +	io_page_cachep = KMEM_CACHE(ext4_io_page, SLAB_RECLAIM_ACCOUNT);
> +	if (io_page_cachep == NULL)
> +		return -ENOMEM;
> +	io_end_cachep = KMEM_CACHE(ext4_io_end, SLAB_RECLAIM_ACCOUNT);
> +	if (io_page_cachep == NULL) {
> +		kmem_cache_destroy(io_page_cachep);
> +		return -ENOMEM;
> +	}
> +
> +	return 0;
> +}
> +
> +void exit_ext4_pageio(void)
> +{
> +	kmem_cache_destroy(io_end_cachep);
> +	kmem_cache_destroy(io_page_cachep);
> +}
> +
> +void ext4_free_io_end(ext4_io_end_t *io)
> +{
> +	int i;
> +
> +	BUG_ON(!io);
> +	if (io->page)
> +		put_page(io->page);
> +	for (i = 0; i < io->num_io_pages; i++) {
> +		if (--io->pages[i]->p_count == 0) {
> +			struct page *page = io->pages[i]->p_page;
> +
> +			end_page_writeback(page);
> +			put_page(page);
> +			kmem_cache_free(io_page_cachep, io->pages[i]);
> +		}
> +	}
> +	io->num_io_pages = 0;
> +	iput(io->inode);
> +	kmem_cache_free(io_end_cachep, io);
> +}
> +
> +/*
> + * check a range of space and convert unwritten extents to written.
> + */
> +int ext4_end_io_nolock(ext4_io_end_t *io)
> +{
> +	struct inode *inode = io->inode;
> +	loff_t offset = io->offset;
> +	ssize_t size = io->size;
> +	int ret = 0;
> +
> +	ext4_debug("ext4_end_io_nolock: io 0x%p from inode %lu,list->next 0x%p,"
> +		   "list->prev 0x%p\n",
> +		   io, inode->i_ino, io->list.next, io->list.prev);
> +
> +	if (list_empty(&io->list))
> +		return ret;
> +
> +	if (!(io->flag & EXT4_IO_END_UNWRITTEN))
> +		return ret;
> +
> +	ret = ext4_convert_unwritten_extents(inode, offset, size);
> +	if (ret < 0) {
> +		printk(KERN_EMERG "%s: failed to convert unwritten "
> +			"extents to written extents, error is %d "
> +			"io is still on inode %lu aio dio list\n",
> +		       __func__, ret, inode->i_ino);
> +		return ret;
> +	}
> +
> +	if (io->iocb)
> +		aio_complete(io->iocb, io->result, 0);
> +	/* clear the DIO AIO unwritten flag */
> +	io->flag &= ~EXT4_IO_END_UNWRITTEN;
> +	return ret;
> +}
> +
> +/*
> + * work on completed aio dio IO, to convert unwritten extents to extents
> + */
> +static void ext4_end_io_work(struct work_struct *work)
> +{
> +	ext4_io_end_t		*io = container_of(work, ext4_io_end_t, work);
> +	struct inode		*inode = io->inode;
> +	struct ext4_inode_info	*ei = EXT4_I(inode);
> +	unsigned long		flags;
> +	int			ret;
> +
> +	mutex_lock(&inode->i_mutex);
> +	ret = ext4_end_io_nolock(io);
> +	if (ret < 0) {
> +		mutex_unlock(&inode->i_mutex);
> +		return;
> +	}
> +
> +	spin_lock_irqsave(&ei->i_completed_io_lock, flags);
> +	if (!list_empty(&io->list))
> +		list_del_init(&io->list);
> +	spin_unlock_irqrestore(&ei->i_completed_io_lock, flags);
> +	mutex_unlock(&inode->i_mutex);
> +	ext4_free_io_end(io);
> +}
> +
> +ext4_io_end_t *ext4_init_io_end(struct inode *inode, gfp_t flags)
> +{
> +	ext4_io_end_t *io = NULL;
> +
> +	io = kmem_cache_alloc(io_end_cachep, flags);
> +	if (io) {
> +		memset(io, 0, sizeof(*io));
> +		io->inode = igrab(inode);
> +		BUG_ON(!io->inode);
> +		INIT_WORK(&io->work, ext4_end_io_work);
> +		INIT_LIST_HEAD(&io->list);
> +	}
> +	return io;
> +}
> +
> +/*
> + * Print an buffer I/O error compatible with the fs/buffer.c.  This
> + * provides compatibility with dmesg scrapers that look for a specific
> + * buffer I/O error message.  We really need a unified error reporting
> + * structure to userspace ala Digital Unix's uerf system, but it's
> + * probably not going to happen in my lifetime, due to LKML politics...
> + */
> +static void buffer_io_error(struct buffer_head *bh)
> +{
> +	char b[BDEVNAME_SIZE];
> +	printk(KERN_ERR "Buffer I/O error on device %s, logical block %llu\n",
> +			bdevname(bh->b_bdev, b),
> +			(unsigned long long)bh->b_blocknr);
> +}
> +
> +static void ext4_end_bio(struct bio *bio, int error)
> +{
> +	ext4_io_end_t *io_end = bio->bi_private;
> +	struct workqueue_struct *wq;
> +	struct inode *inode;
> +	unsigned long flags;
> +	int i;
> +
> +	BUG_ON(!io_end);
> +	inode = io_end->inode;
> +	bio->bi_private = NULL;
> +	bio->bi_end_io = NULL;
> +	if (test_bit(BIO_UPTODATE, &bio->bi_flags))
> +		error = 0;
> +	bio_put(bio);
> +
> +	if (!(inode->i_sb->s_flags & MS_ACTIVE)) {
> +		pr_err("sb umounted, discard end_io request for inode %lu\n",
> +			io_end->inode->i_ino);
> +		ext4_free_io_end(io_end);
> +		return;
> +	}
> +
> +	if (error) {
> +		io_end->flag |= EXT4_IO_END_ERROR;
> +		ext4_warning(inode->i_sb, "I/O error writing inode %lu "
> +			     "(offset %llu size %ld)", inode->i_ino,
> +			     (unsigned long long) io_end->offset,
> +			     (long) io_end->size);
> +	}
> +
> +	for (i = 0; i < io_end->num_io_pages; i++) {
> +		struct page *page = io_end->pages[i]->p_page;
> +		struct buffer_head *bh, *head;
> +		int partial_write = 0;
> +
> +		head = page_buffers(page);
> +		if (error)
> +			SetPageError(page);
> +		BUG_ON(!head);
> +		if (head->b_size == PAGE_CACHE_SIZE)
> +			clear_buffer_dirty(head);
> +		else {
> +			loff_t offset;
> +			loff_t io_end_offset = io_end->offset + io_end->size;
> +
> +			offset = (sector_t) page->index << PAGE_CACHE_SHIFT;
> +			bh = head;
> +			do {
> +				if ((offset >= io_end->offset) &&
> +				    (offset+bh->b_size <= io_end_offset)) {
> +					if (error)
> +						buffer_io_error(bh);
> +
> +					clear_buffer_dirty(bh);
> +				}
> +				if (buffer_delay(bh))
> +					partial_write = 1;
> +				else if (!buffer_mapped(bh))
> +					clear_buffer_dirty(bh);
> +				else if (buffer_dirty(bh))
> +					partial_write = 1;
> +				offset += bh->b_size;
> +				bh = bh->b_this_page;
> +			} while (bh != head);
> +		}
> +
> +		if (--io_end->pages[i]->p_count == 0) {
> +			struct page *page = io_end->pages[i]->p_page;
> +
> +			end_page_writeback(page);
> +			put_page(page);
> +			kmem_cache_free(io_page_cachep, io_end->pages[i]);
> +		}
> +
> +		/*
> +		 * If this is a partial write which happened to make
> +		 * all buffers uptodate then we can optimize away a
> +		 * bogus readpage() for the next read(). Here we
> +		 * 'discover' whether the page went uptodate as a
> +		 * result of this (potentially partial) write.
> +		 */
> +		if (!partial_write)
> +			SetPageUptodate(page);
> +	}
> +
> +	io_end->num_io_pages = 0;
> +
> +	/* Add the io_end to per-inode completed io list*/
> +	spin_lock_irqsave(&EXT4_I(inode)->i_completed_io_lock, flags);
> +	list_add_tail(&io_end->list, &EXT4_I(inode)->i_completed_io_list);
> +	spin_unlock_irqrestore(&EXT4_I(inode)->i_completed_io_lock, flags);
> +
> +	wq = EXT4_SB(inode->i_sb)->dio_unwritten_wq;
> +	/* queue the work to convert unwritten extents to written */
> +	queue_work(wq, &io_end->work);
> +}
> +
> +void ext4_io_submit(struct ext4_io_submit *io)
> +{
> +	struct bio *bio = io->io_bio;
> +
> +	if (bio) {
> +		bio_get(io->io_bio);
> +		submit_bio(io->io_op, io->io_bio);
> +		BUG_ON(bio_flagged(io->io_bio, BIO_EOPNOTSUPP));
Definitly this BUG_ON should be converted to ext4_error or something
similar, otherwhise writeback attempt to removed usb-stick will be fatal
for a whole system. IMHO it is reasonable to skip this check at all,
because all work will be done in ext4_end_bio() anyway.
> +		bio_put(io->io_bio);
> +	}
> +	io->io_bio = 0;
> +	io->io_op = 0;
> +	io->io_end = 0;
> +}
> +
> +static int io_submit_init(struct ext4_io_submit *io,
> +			  struct inode *inode,
> +			  struct writeback_control *wbc,
> +			  struct buffer_head *bh)
> +{
> +	ext4_io_end_t *io_end;
> +	struct page *page = bh->b_page;
> +	int nvecs = bio_get_nr_vecs(bh->b_bdev);
> +	struct bio *bio;
> +
> +	io_end = ext4_init_io_end(inode, GFP_NOFS);
> +	if (!io_end)
> +		return -ENOMEM;
> +	do {
> +		bio = bio_alloc(GFP_NOIO, nvecs);
> +		nvecs >>= 1;
> +	} while (bio == NULL);
> +
> +	bio->bi_sector = bh->b_blocknr * (bh->b_size >> 9);
> +	bio->bi_bdev = bh->b_bdev;
> +	bio->bi_private = io->io_end = io_end;
> +	bio->bi_end_io = ext4_end_bio;
> +
> +	io_end->inode = inode;
> +	io_end->offset = (page->index << PAGE_CACHE_SHIFT) + bh_offset(bh);
> +
> +	io->io_bio = bio;
> +	io->io_op = (wbc->sync_mode == WB_SYNC_ALL ?
> +			WRITE_SYNC_PLUG : WRITE);
> +	io->io_next_block = bh->b_blocknr;
> +	return 0;
> +}
> +
> +static int io_submit_add_bh(struct ext4_io_submit *io,
> +			    struct ext4_io_page *io_page,
> +			    struct inode *inode,
> +			    struct writeback_control *wbc,
> +			    struct buffer_head *bh)
> +{
> +	ext4_io_end_t *io_end;
> +	int ret;
> +
> +	if (buffer_new(bh)) {
> +		clear_buffer_new(bh);
> +		unmap_underlying_metadata(bh->b_bdev, bh->b_blocknr);
> +	}
> +
> +	if (!buffer_mapped(bh) || buffer_delay(bh)) {
> +		if (!buffer_mapped(bh))
> +			clear_buffer_dirty(bh);
> +		if (io->io_bio)
> +			ext4_io_submit(io);
> +		return 0;
> +	}
> +
> +	if (io->io_bio && bh->b_blocknr != io->io_next_block) {
> +submit_and_retry:
> +		ext4_io_submit(io);
> +	}
> +	if (io->io_bio == NULL) {
> +		ret = io_submit_init(io, inode, wbc, bh);
> +		if (ret)
> +			return ret;
> +	}
> +	io_end = io->io_end;
> +	if ((io_end->num_io_pages >= MAX_IO_PAGES) &&
> +	    (io_end->pages[io_end->num_io_pages-1] != io_page))
> +		goto submit_and_retry;
> +	if (buffer_uninit(bh))
> +		io->io_end->flag |= EXT4_IO_END_UNWRITTEN;
> +	io->io_end->size += bh->b_size;
> +	io->io_next_block++;
> +	ret = bio_add_page(io->io_bio, bh->b_page, bh->b_size, bh_offset(bh));
> +	if (ret != bh->b_size)
> +		goto submit_and_retry;
> +	if ((io_end->num_io_pages == 0) ||
> +	    (io_end->pages[io_end->num_io_pages-1] != io_page)) {
> +		io_end->pages[io_end->num_io_pages++] = io_page;
> +		io_page->p_count++;
> +	}
> +	return 0;
> +}
> +
> +int ext4_bio_write_page(struct ext4_io_submit *io,
> +			struct page *page,
> +			int len,
> +			struct writeback_control *wbc)
> +{
> +	struct inode *inode = page->mapping->host;
> +	unsigned block_start, block_end, blocksize;
> +	struct ext4_io_page *io_page;
> +	struct buffer_head *bh, *head;
> +	int ret = 0;
> +
> +	blocksize = 1 << inode->i_blkbits;
> +
> +	BUG_ON(PageWriteback(page));
> +	set_page_writeback(page);
> +	ClearPageError(page);
> +
> +	io_page = kmem_cache_alloc(io_page_cachep, GFP_NOFS);
> +	if (!io_page) {
> +		set_page_dirty(page);
> +		unlock_page(page);
> +		return -ENOMEM;
> +	}
> +	io_page->p_page = page;
> +	io_page->p_count = 0;
> +	get_page(page);
> +
> +	for (bh = head = page_buffers(page), block_start = 0;
> +	     bh != head || !block_start;
> +	     block_start = block_end, bh = bh->b_this_page) {
> +		block_end = block_start + blocksize;
> +		if (block_start >= len) {
> +			clear_buffer_dirty(bh);
> +			set_buffer_uptodate(bh);
> +			continue;
> +		}
> +		ret = io_submit_add_bh(io, io_page, inode, wbc, bh);
> +		if (ret) {
> +			/*
> +			 * We only get here on ENOMEM.  Not much else
> +			 * we can do but mark the page as dirty, and
> +			 * better luck next time.
> +			 */
> +			set_page_dirty(page);
> +			break;
> +		}
> +	}
> +	unlock_page(page);
> +	/*
> +	 * If the page was truncated before we could do the writeback,
> +	 * or we had a memory allocation error while trying to write
> +	 * the first buffer head, we won't have submitted any pages for
> +	 * I/O.  In that case we need to make sure we've cleared the
> +	 * PageWriteback bit from the page to prevent the system from
> +	 * wedging later on.
> +	 */
> +	if (io_page->p_count == 0) {
> +		put_page(page);
> +		end_page_writeback(page);
> +		kmem_cache_free(io_page_cachep, io_page);
> +	}
> +	return ret;
> +}
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 16002ec..9f602c2 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -4768,9 +4768,12 @@ static int __init init_ext4_fs(void)
>  	int err;
>  
>  	ext4_check_flag_values();
> -	err = init_ext4_system_zone();
> +	err = init_ext4_pageio();
>  	if (err)
>  		return err;
> +	err = init_ext4_system_zone();
> +	if (err)
> +		goto out5;
>  	ext4_kset = kset_create_and_add("ext4", NULL, fs_kobj);
>  	if (!ext4_kset)
>  		goto out4;
> @@ -4811,6 +4814,8 @@ out3:
>  	kset_unregister(ext4_kset);
>  out4:
>  	exit_ext4_system_zone();
> +out5:
> +	exit_ext4_pageio();
>  	return err;
>  }
>  
> @@ -4826,6 +4831,7 @@ static void __exit exit_ext4_fs(void)
>  	remove_proc_entry("fs/ext4", NULL);
>  	kset_unregister(ext4_kset);
>  	exit_ext4_system_zone();
> +	exit_ext4_pageio();
>  }
>  
>  MODULE_AUTHOR("Remy Card, Stephen Tweedie, Andrew Morton, Andreas Dilger, Theodore Ts'o and others");
> -- 
> 1.7.1
> 
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