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

Darrick J. Wong
2020-10-29T21:45:33Z

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
>