Re: [PATCH 01/29] ext4: Make ext4_bio_write_page() use BH_Async_Write flags instead page pointers from ext4_io_end

[Dmitry Monakhov <dmonakhov@openvz.org>]

On Mon,  8 Apr 2013 23:32:06 +0200, Jan Kara <jack@suse.cz> wrote:
> So far ext4_bio_write_page() attached all the pages to ext4_io_end structure.
> This makes that structure pretty heavy (1 KB for pointers + 16 bytes per
> page attached to the bio). Also later we would like to share ext4_io_end
> structure among several bios in case IO to a single extent needs to be split
> among several bios and pointing to pages from ext4_io_end makes this complex.
> 
> We remove page pointers from ext4_io_end and use pointers from bio itself
> instead. This isn't as easy when blocksize < pagesize because then we can have
> several bios in flight for a single page and we have to be careful when to call
> end_page_writeback(). However this is a known problem already solved by
> block_write_full_page() / end_buffer_async_write() so we mimic its behavior
> here. We mark buffers going to disk with BH_Async_Write flag and in
> ext4_bio_end_io() we check whether there are any buffers with BH_Async_Write
> flag left. If there are not, we can call end_page_writeback().
Definitely looks better than my semi-fix.
Reviewed-by: Dmitry Monakhov <dmonakhov@openvz.org>
> 
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  fs/ext4/ext4.h    |   14 -----
>  fs/ext4/page-io.c |  163 +++++++++++++++++++++++++----------------------------
>  2 files changed, 77 insertions(+), 100 deletions(-)
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 4a01ba3..3c70547 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -196,19 +196,8 @@ struct mpage_da_data {
>  #define EXT4_IO_END_ERROR	0x0002
>  #define EXT4_IO_END_DIRECT	0x0004
>  
> -struct ext4_io_page {
> -	struct page	*p_page;
> -	atomic_t	p_count;
> -};
> -
> -#define MAX_IO_PAGES 128
> -
>  /*
>   * For converting uninitialized extents on a work queue.
> - *
> - * 'page' is only used from the writepage() path; 'pages' is only used for
> - * buffered writes; they are used to keep page references until conversion
> - * takes place.  For AIO/DIO, neither field is filled in.
>   */
>  typedef struct ext4_io_end {
>  	struct list_head	list;		/* per-file finished IO list */
> @@ -218,15 +207,12 @@ typedef struct ext4_io_end {
>  	ssize_t			size;		/* size of the extent */
>  	struct kiocb		*iocb;		/* iocb struct for AIO */
>  	int			result;		/* error value for AIO */
> -	int			num_io_pages;   /* for writepages() */
> -	struct ext4_io_page	*pages[MAX_IO_PAGES]; /* for writepages() */
>  } 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;
>  };
>  
> diff --git a/fs/ext4/page-io.c b/fs/ext4/page-io.c
> index 809b310..73bc011 100644
> --- a/fs/ext4/page-io.c
> +++ b/fs/ext4/page-io.c
> @@ -29,25 +29,19 @@
>  #include "xattr.h"
>  #include "acl.h"
>  
> -static struct kmem_cache *io_page_cachep, *io_end_cachep;
> +static struct kmem_cache *io_end_cachep;
>  
>  int __init ext4_init_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_end_cachep == NULL) {
> -		kmem_cache_destroy(io_page_cachep);
> +	if (io_end_cachep == NULL)
>  		return -ENOMEM;
> -	}
>  	return 0;
>  }
>  
>  void ext4_exit_pageio(void)
>  {
>  	kmem_cache_destroy(io_end_cachep);
> -	kmem_cache_destroy(io_page_cachep);
>  }
>  
>  void ext4_ioend_wait(struct inode *inode)
> @@ -57,15 +51,6 @@ void ext4_ioend_wait(struct inode *inode)
>  	wait_event(*wq, (atomic_read(&EXT4_I(inode)->i_ioend_count) == 0));
>  }
>  
> -static void put_io_page(struct ext4_io_page *io_page)
> -{
> -	if (atomic_dec_and_test(&io_page->p_count)) {
> -		end_page_writeback(io_page->p_page);
> -		put_page(io_page->p_page);
> -		kmem_cache_free(io_page_cachep, io_page);
> -	}
> -}
> -
>  void ext4_free_io_end(ext4_io_end_t *io)
>  {
>  	int i;
> @@ -74,9 +59,6 @@ void ext4_free_io_end(ext4_io_end_t *io)
>  	BUG_ON(!list_empty(&io->list));
>  	BUG_ON(io->flag & EXT4_IO_END_UNWRITTEN);
>  
> -	for (i = 0; i < io->num_io_pages; i++)
> -		put_io_page(io->pages[i]);
> -	io->num_io_pages = 0;
>  	if (atomic_dec_and_test(&EXT4_I(io->inode)->i_ioend_count))
>  		wake_up_all(ext4_ioend_wq(io->inode));
>  	kmem_cache_free(io_end_cachep, io);
> @@ -233,45 +215,56 @@ static void ext4_end_bio(struct bio *bio, int error)
>  	ext4_io_end_t *io_end = bio->bi_private;
>  	struct inode *inode;
>  	int i;
> +	int blocksize;
>  	sector_t bi_sector = bio->bi_sector;
>  
>  	BUG_ON(!io_end);
> +	inode = io_end->inode;
> +	blocksize = 1 << inode->i_blkbits;
>  	bio->bi_private = NULL;
>  	bio->bi_end_io = NULL;
>  	if (test_bit(BIO_UPTODATE, &bio->bi_flags))
>  		error = 0;
> -	bio_put(bio);
> -
> -	for (i = 0; i < io_end->num_io_pages; i++) {
> -		struct page *page = io_end->pages[i]->p_page;
> +	for (i = 0; i < bio->bi_vcnt; i++) {
> +		struct bio_vec *bvec = &bio->bi_io_vec[i];
> +		struct page *page = bvec->bv_page;
>  		struct buffer_head *bh, *head;
> -		loff_t offset;
> -		loff_t io_end_offset;
> +		unsigned bio_start = bvec->bv_offset;
> +		unsigned bio_end = bio_start + bvec->bv_len;
> +		unsigned under_io = 0;
> +		unsigned long flags;
> +
> +		if (!page)
> +			continue;
>  
>  		if (error) {
>  			SetPageError(page);
>  			set_bit(AS_EIO, &page->mapping->flags);
> -			head = page_buffers(page);
> -			BUG_ON(!head);
> -
> -			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))
> -					buffer_io_error(bh);
> -
> -				offset += bh->b_size;
> -				bh = bh->b_this_page;
> -			} while (bh != head);
>  		}
> -
> -		put_io_page(io_end->pages[i]);
> +		bh = head = page_buffers(page);
> +		/*
> +		 * We check all buffers in the page under BH_Uptodate_Lock
> +		 * to avoid races with other end io clearing async_write flags
> +		 */
> +		local_irq_save(flags);
> +		bit_spin_lock(BH_Uptodate_Lock, &head->b_state);
> +		do {
> +			if (bh_offset(bh) < bio_start ||
> +			    bh_offset(bh) + blocksize > bio_end) {
> +				if (buffer_async_write(bh))
> +					under_io++;
> +				continue;
> +			}
> +			clear_buffer_async_write(bh);
> +			if (error)
> +				buffer_io_error(bh);
> +		} while ((bh = bh->b_this_page) != head);
> +		bit_spin_unlock(BH_Uptodate_Lock, &head->b_state);
> +		local_irq_restore(flags);
> +		if (!under_io)
> +			end_page_writeback(page);
>  	}
> -	io_end->num_io_pages = 0;
> -	inode = io_end->inode;
> +	bio_put(bio);
>  
>  	if (error) {
>  		io_end->flag |= EXT4_IO_END_ERROR;
> @@ -335,7 +328,6 @@ static int io_submit_init(struct ext4_io_submit *io,
>  }
>  
>  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)
> @@ -343,11 +335,6 @@ static int io_submit_add_bh(struct ext4_io_submit *io,
>  	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 (io->io_bio && bh->b_blocknr != io->io_next_block) {
>  submit_and_retry:
>  		ext4_io_submit(io);
> @@ -358,9 +345,6 @@ submit_and_retry:
>  			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))
>  		ext4_set_io_unwritten_flag(inode, io_end);
>  	io->io_end->size += bh->b_size;
> @@ -368,11 +352,6 @@ submit_and_retry:
>  	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;
> -		atomic_inc(&io_page->p_count);
> -	}
>  	return 0;
>  }
>  
> @@ -382,33 +361,29 @@ int ext4_bio_write_page(struct ext4_io_submit *io,
>  			struct writeback_control *wbc)
>  {
>  	struct inode *inode = page->mapping->host;
> -	unsigned block_start, block_end, blocksize;
> -	struct ext4_io_page *io_page;
> +	unsigned block_start, blocksize;
>  	struct buffer_head *bh, *head;
>  	int ret = 0;
> +	int nr_submitted = 0;
>  
>  	blocksize = 1 << inode->i_blkbits;
>  
>  	BUG_ON(!PageLocked(page));
>  	BUG_ON(PageWriteback(page));
>  
> -	io_page = kmem_cache_alloc(io_page_cachep, GFP_NOFS);
> -	if (!io_page) {
> -		redirty_page_for_writepage(wbc, page);
> -		unlock_page(page);
> -		return -ENOMEM;
> -	}
> -	io_page->p_page = page;
> -	atomic_set(&io_page->p_count, 1);
> -	get_page(page);
>  	set_page_writeback(page);
>  	ClearPageError(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;
> +	/*
> +	 * In the first loop we prepare and mark buffers to submit. We have to
> +	 * mark all buffers in the page before submitting so that
> +	 * end_page_writeback() cannot be called from ext4_bio_end_io() when IO
> +	 * on the first buffer finishes and we are still working on submitting
> +	 * the second buffer.
> +	 */
> +	bh = head = page_buffers(page);
> +	do {
> +		block_start = bh_offset(bh);
>  		if (block_start >= len) {
>  			/*
>  			 * Comments copied from block_write_full_page_endio:
> @@ -421,7 +396,8 @@ int ext4_bio_write_page(struct ext4_io_submit *io,
>  			 * mapped, and writes to that region are not written
>  			 * out to the file."
>  			 */
> -			zero_user_segment(page, block_start, block_end);
> +			zero_user_segment(page, block_start,
> +					  block_start + blocksize);
>  			clear_buffer_dirty(bh);
>  			set_buffer_uptodate(bh);
>  			continue;
> @@ -435,7 +411,19 @@ int ext4_bio_write_page(struct ext4_io_submit *io,
>  				ext4_io_submit(io);
>  			continue;
>  		}
> -		ret = io_submit_add_bh(io, io_page, inode, wbc, bh);
> +		if (buffer_new(bh)) {
> +			clear_buffer_new(bh);
> +			unmap_underlying_metadata(bh->b_bdev, bh->b_blocknr);
> +		}
> +		set_buffer_async_write(bh);
> +	} while ((bh = bh->b_this_page) != head);
> +
> +	/* Now submit buffers to write */
> +	bh = head = page_buffers(page);
> +	do {
> +		if (!buffer_async_write(bh))
> +			continue;
> +		ret = io_submit_add_bh(io, inode, wbc, bh);
>  		if (ret) {
>  			/*
>  			 * We only get here on ENOMEM.  Not much else
> @@ -445,17 +433,20 @@ int ext4_bio_write_page(struct ext4_io_submit *io,
>  			redirty_page_for_writepage(wbc, page);
>  			break;
>  		}
> +		nr_submitted++;
>  		clear_buffer_dirty(bh);
> +	} while ((bh = bh->b_this_page) != head);
> +
> +	/* Error stopped previous loop? Clean up buffers... */
> +	if (ret) {
> +		do {
> +			clear_buffer_async_write(bh);
> +			bh = bh->b_this_page;
> +		} while (bh != head);
>  	}
>  	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.
> -	 */
> -	put_io_page(io_page);
> +	/* Nothing submitted - we have to end page writeback */
> +	if (!nr_submitted)
> +		end_page_writeback(page);
>  	return ret;
>  }
> -- 
> 1.7.1
> 
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