Re: [PATCH 3/9] xfs: punching delalloc extents on write failure is racy

Darrick J. Wong
2022-11-15T23:53:50Z

On Tue, Nov 15, 2022 at 12:30:37PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> xfs_buffered_write_iomap_end() has a comment about the safety of
> punching delalloc extents based holding the IOLOCK_EXCL. This
> comment is wrong, and punching delalloc extents is not race free.
>
> When we punch out a delalloc extent after a write failure in
> xfs_buffered_write_iomap_end(), we punch out the page cache with
> truncate_pagecache_range() before we punch out the delalloc extents.
> At this point, we only hold the IOLOCK_EXCL, so there is nothing
> stopping mmap() write faults racing with this cleanup operation,
> reinstantiating a folio over the range we are about to punch and
> hence requiring the delalloc extent to be kept.
>
> If this race condition is hit, we can end up with a dirty page in
> the page cache that has no delalloc extent or space reservation
> backing it. This leads to bad things happening at writeback time.
>
> To avoid this race condition, we need the page cache truncation to
> be atomic w.r.t. the extent manipulation. We can do this by holding
> the mapping->invalidate_lock exclusively across this operation -
> this will prevent new pages from being inserted into the page cache
> whilst we are removing the pages and the backing extent and space
> reservation.
>
> Taking the mapping->invalidate_lock exclusively in the buffered
> write IO path is safe - it naturally nests inside the IOLOCK (see
> truncate and fallocate paths). iomap_zero_range() can be called from
> under the mapping->invalidate_lock (from the truncate path via
> either xfs_zero_eof() or xfs_truncate_page(), but iomap_zero_iter()
> will not instantiate new delalloc pages (because it skips holes) and
> hence will not ever need to punch out delalloc extents on failure.
>
> Fix the locking issue, and clean up the code logic a little to avoid
> unnecessary work if we didn't allocate the delalloc extent or wrote
> the entire region we allocated.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>

Didn't I already tag this...?

Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> ---
> fs/xfs/xfs_iomap.c | 41 +++++++++++++++++++++++------------------
> 1 file changed, 23 insertions(+), 18 deletions(-)
>
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index 5cea069a38b4..a2e45ea1b0cb 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -1147,6 +1147,10 @@ xfs_buffered_write_iomap_end(
> written = 0;
> }
>
> + /* If we didn't reserve the blocks, we're not allowed to punch them. */
> + if (!(iomap->flags & IOMAP_F_NEW))
> + return 0;
> +
> /*
> * start_fsb refers to the first unused block after a short write. If
> * nothing was written, round offset down to point at the first block in
> @@ -1158,27 +1162,28 @@ xfs_buffered_write_iomap_end(
> start_fsb = XFS_B_TO_FSB(mp, offset + written);
> end_fsb = XFS_B_TO_FSB(mp, offset + length);
>
> + /* Nothing to do if we've written the entire delalloc extent */
> + if (start_fsb >= end_fsb)
> + return 0;
> +
> /*
> - * Trim delalloc blocks if they were allocated by this write and we
> - * didn't manage to write the whole range.
> - *
> - * We don't need to care about racing delalloc as we hold i_mutex
> - * across the reserve/allocate/unreserve calls. If there are delalloc
> - * blocks in the range, they are ours.
> + * Lock the mapping to avoid races with page faults re-instantiating
> + * folios and dirtying them via ->page_mkwrite between the page cache
> + * truncation and the delalloc extent removal. Failing to do this can
> + * leave dirty pages with no space reservation in the cache.
> */
> - if ((iomap->flags & IOMAP_F_NEW) && start_fsb < end_fsb) {
> - truncate_pagecache_range(VFS_I(ip), XFS_FSB_TO_B(mp, start_fsb),
> - XFS_FSB_TO_B(mp, end_fsb) - 1);
> -
> - error = xfs_bmap_punch_delalloc_range(ip, start_fsb,
> - end_fsb - start_fsb);
> - if (error && !xfs_is_shutdown(mp)) {
> - xfs_alert(mp, "%s: unable to clean up ino %lld",
> - __func__, ip->i_ino);
> - return error;
> - }
> + filemap_invalidate_lock(inode->i_mapping);
> + truncate_pagecache_range(VFS_I(ip), XFS_FSB_TO_B(mp, start_fsb),
> + XFS_FSB_TO_B(mp, end_fsb) - 1);
> +
> + error = xfs_bmap_punch_delalloc_range(ip, start_fsb,
> + end_fsb - start_fsb);
> + filemap_invalidate_unlock(inode->i_mapping);
> + if (error && !xfs_is_shutdown(mp)) {
> + xfs_alert(mp, "%s: unable to clean up ino %lld",
> + __func__, ip->i_ino);
> + return error;
> }
> -
> return 0;
> }
>
> --
> 2.37.2
>