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From: "Darrick J. Wong" <djwong@kernel.org>
To: Dave Chinner <david@fromorbit.com>, Matthew Wilcox <willy@infradead.org>
Cc: linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	xfs <linux-xfs@vger.kernel.org>,
	Christoph Hellwig <hch@infradead.org>
Subject: [PATCH] iomap: don't invalidate folios after writeback errors
Date: Sun, 15 May 2022 20:37:09 -0700	[thread overview]
Message-ID: <YoHG5cMwvx8PSddI@magnolia> (raw)

From: Darrick J. Wong <djwong@kernel.org>

XFS has the unique behavior (as compared to the other Linux filesystems)
that on writeback errors it will completely invalidate the affected
folio and force the page cache to reread the contents from disk.  All
other filesystems leave the page mapped and up to date.

This is a rude awakening for user programs, since (in the case where
write fails but reread doesn't) file contents will appear to revert to
old disk contents with no notification other than an EIO on fsync.  This
might have been annoying back in the days when iomap dealt with one page
at a time, but with multipage folios, we can now throw away *megabytes*
worth of data for a single write error.

On *most* Linux filesystems, a program can respond to an EIO on write by
redirtying the entire file and scheduling it for writeback.  This isn't
foolproof, since the page that failed writeback is no longer dirty and
could be evicted, but programs that want to recover properly *also*
have to detect XFS and regenerate every write they've made to the file.

When running xfs/314 on arm64, I noticed a UAF bug when xfs_discard_folio
invalidates multipage folios that could be undergoing writeback.  If,
say, we have a 256K folio caching a mix of written and unwritten
extents, it's possible that we could start writeback of the first (say)
64K of the folio and then hit a writeback error on the next 64K.  We
then free the iop attached to the folio, which is really bad because
writeback completion on the first 64k will trip over the "blocks per
folio > 1 && !iop" assertion.

This can't be fixed by only invalidating the folio if writeback fails at
the start of the folio, since the folio is marked !uptodate, which trips
other assertions elsewhere.  Get rid of the whole behavior entirely.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
---
 fs/iomap/buffered-io.c |    1 -
 fs/xfs/xfs_aops.c      |    4 +---
 2 files changed, 1 insertion(+), 4 deletions(-)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index 8fb9b2797fc5..94b53cbdefad 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -1387,7 +1387,6 @@ iomap_writepage_map(struct iomap_writepage_ctx *wpc,
 		if (wpc->ops->discard_folio)
 			wpc->ops->discard_folio(folio, pos);
 		if (!count) {
-			folio_clear_uptodate(folio);
 			folio_unlock(folio);
 			goto done;
 		}
diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
index 90b7f4d127de..f6216d0fb0c2 100644
--- a/fs/xfs/xfs_aops.c
+++ b/fs/xfs/xfs_aops.c
@@ -464,7 +464,7 @@ xfs_discard_folio(
 	int			error;
 
 	if (xfs_is_shutdown(mp))
-		goto out_invalidate;
+		return;
 
 	xfs_alert_ratelimited(mp,
 		"page discard on page "PTR_FMT", inode 0x%llx, pos %llu.",
@@ -474,8 +474,6 @@ xfs_discard_folio(
 			i_blocks_per_folio(inode, folio) - pageoff_fsb);
 	if (error && !xfs_is_shutdown(mp))
 		xfs_alert(mp, "page discard unable to remove delalloc mapping.");
-out_invalidate:
-	iomap_invalidate_folio(folio, offset, folio_size(folio) - offset);
 }
 
 static const struct iomap_writeback_ops xfs_writeback_ops = {

             reply	other threads:[~2022-05-16  3:37 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-05-16  3:37 Darrick J. Wong [this message]
2022-05-16 12:27 ` Matthew Wilcox
2022-05-16 12:53 ` Jeff Layton
2022-05-16 13:43 ` Christoph Hellwig

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