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List:       linux-xfs
Subject:    Re: [PATCH v2.1 2/3] iomap: support partial page discard on writeback block mapping failure
From:       Allison Henderson <allison.henderson () oracle ! com>
Date:       2020-10-30 23:23:49
Message-ID: 6abde9e8-c701-cf66-254c-b2711eab0346 () oracle ! com
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On 10/29/20 9:33 AM, Brian Foster wrote:
> iomap writeback mapping failure only calls into ->discard_page() if
> the current page has not been added to the ioend. Accordingly, the
> XFS callback assumes a full page discard and invalidation. This is
> problematic for sub-page block size filesystems where some portion
> of a page might have been mapped successfully before a failure to
> map a delalloc block occurs. ->discard_page() is not called in that
> error scenario and the bio is explicitly failed by iomap via the
> error return from ->prepare_ioend(). As a result, the filesystem
> leaks delalloc blocks and corrupts the filesystem block counters.
> 
> Since XFS is the only user of ->discard_page(), tweak the semantics
> to invoke the callback unconditionally on mapping errors and provide
> the file offset that failed to map. Update xfs_discard_page() to
> discard the corresponding portion of the file and pass the range
> along to iomap_invalidatepage(). The latter already properly handles
> both full and sub-page scenarios by not changing any iomap or page
> state on sub-page invalidations.
> 
Looks ok to me
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>

> Signed-off-by: Brian Foster <bfoster@redhat.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> ---
> 
> v2.1:
> - Rebased to v5.10-rc1.
> 
>   fs/iomap/buffered-io.c | 15 ++++++++-------
>   fs/xfs/xfs_aops.c      | 14 ++++++++------
>   include/linux/iomap.h  |  2 +-
>   3 files changed, 17 insertions(+), 14 deletions(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 8180061b9e16..e4ea1f9f94d0 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -1382,14 +1382,15 @@ iomap_writepage_map(struct iomap_writepage_ctx *wpc,
>   	 * appropriately.
>   	 */
>   	if (unlikely(error)) {
> +		/*
> +		 * Let the filesystem know what portion of the current page
> +		 * failed to map. If the page wasn't been added to ioend, it
> +		 * won't be affected by I/O completion and we must unlock it
> +		 * now.
> +		 */
> +		if (wpc->ops->discard_page)
> +			wpc->ops->discard_page(page, file_offset);
>   		if (!count) {
> -			/*
> -			 * If the current page hasn't been added to ioend, it
> -			 * won't be affected by I/O completions and we must
> -			 * discard and unlock it right here.
> -			 */
> -			if (wpc->ops->discard_page)
> -				wpc->ops->discard_page(page);
>   			ClearPageUptodate(page);
>   			unlock_page(page);
>   			goto done;
> diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
> index 55d126d4e096..5bf37afae5e9 100644
> --- a/fs/xfs/xfs_aops.c
> +++ b/fs/xfs/xfs_aops.c
> @@ -527,13 +527,15 @@ xfs_prepare_ioend(
>    */
>   static void
>   xfs_discard_page(
> -	struct page		*page)
> +	struct page		*page,
> +	loff_t			fileoff)
>   {
>   	struct inode		*inode = page->mapping->host;
>   	struct xfs_inode	*ip = XFS_I(inode);
>   	struct xfs_mount	*mp = ip->i_mount;
> -	loff_t			offset = page_offset(page);
> -	xfs_fileoff_t		start_fsb = XFS_B_TO_FSBT(mp, offset);
> +	unsigned int		pageoff = offset_in_page(fileoff);
> +	xfs_fileoff_t		start_fsb = XFS_B_TO_FSBT(mp, fileoff);
> +	xfs_fileoff_t		pageoff_fsb = XFS_B_TO_FSBT(mp, pageoff);
>   	int			error;
>   
>   	if (XFS_FORCED_SHUTDOWN(mp))
> @@ -541,14 +543,14 @@ xfs_discard_page(
>   
>   	xfs_alert_ratelimited(mp,
>   		"page discard on page "PTR_FMT", inode 0x%llx, offset %llu.",
> -			page, ip->i_ino, offset);
> +			page, ip->i_ino, fileoff);
>   
>   	error = xfs_bmap_punch_delalloc_range(ip, start_fsb,
> -			i_blocks_per_page(inode, page));
> +			i_blocks_per_page(inode, page) - pageoff_fsb);
>   	if (error && !XFS_FORCED_SHUTDOWN(mp))
>   		xfs_alert(mp, "page discard unable to remove delalloc mapping.");
>   out_invalidate:
> -	iomap_invalidatepage(page, 0, PAGE_SIZE);
> +	iomap_invalidatepage(page, pageoff, PAGE_SIZE - pageoff);
>   }
>   
>   static const struct iomap_writeback_ops xfs_writeback_ops = {
> diff --git a/include/linux/iomap.h b/include/linux/iomap.h
> index 172b3397a1a3..5bd3cac4df9c 100644
> --- a/include/linux/iomap.h
> +++ b/include/linux/iomap.h
> @@ -221,7 +221,7 @@ struct iomap_writeback_ops {
>   	 * Optional, allows the file system to discard state on a page where
>   	 * we failed to submit any I/O.
>   	 */
> -	void (*discard_page)(struct page *page);
> +	void (*discard_page)(struct page *page, loff_t fileoff);
>   };
>   
>   struct iomap_writepage_ctx {
> 
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