Re: [PATCH v2.1 2/3] iomap: support partial page discard on writeback block mapping failure

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Thu, Oct 29, 2020 at 12:33:13PM -0400, 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.
> 
> Signed-off-by: Brian Foster <bfoster@redhat.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>

Thanks for the rebase,
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> ---
> 
> 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 {
> -- 
> 2.25.4
>