[Devel] [RH7 PATCH] [PATCH] ext4: fix data corruption caused by unwritten and delayed extents

[Dmitry Monakhov]

https://jira.sw.ru/browse/PSBM-40580
Backport patch from linux-stable

linux-stable commit  5a7d1e16a425786d8f6f5d4f707bb4dc879499d1
mainsream commit d2dc317d564a46dfc683978a2e5a4f91434e9711 upstream.

Currently it is possible to lose whole file system block worth of data
when we hit the specific interaction with unwritten and delayed extents
in status extent tree.

The problem is that when we insert delayed extent into extent status
tree the only way to get rid of it is when we write out delayed buffer.
However there is a limitation in the extent status tree implementation
so that when inserting unwritten extent should there be even a single
delayed block the whole unwritten extent would be marked as delayed.

At this point, there is no way to get rid of the delayed extents,
because there are no delayed buffers to write out. So when a we write
into said unwritten extent we will convert it to written, but it still
remains delayed.

When we try to write into that block later ext4_da_map_blocks() will set
the buffer new and delayed and map it to invalid block which causes
the rest of the block to be zeroed loosing already written data.

For now we can fix this by simply not allowing to set delayed status on
written extent in the extent status tree. Also add WARN_ON() to make
sure that we notice if this happens in the future.

This problem can be easily reproduced by running the following xfs_io.

xfs_io -f -c "pwrite -S 0xaa 4096 2048" \
          -c "falloc 0 131072" \
          -c "pwrite -S 0xbb 65536 2048" \
          -c "fsync" /mnt/test/fff

echo 3 > /proc/sys/vm/drop_caches
xfs_io -c "pwrite -S 0xdd 67584 2048" /mnt/test/fff

This can be theoretically also reproduced by at random by running fsx,
but it's not very reliable, though on machines with bigger page size
(like ppc) this can be seen more often (especially xfstest generic/127)

Signed-off-by: Lukas Czerner <lczerner at redhat.com>
Signed-off-by: Theodore Ts'o <tytso at mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
Signed-off-by: Dmitry Monakhov <dmonakhov at openvz.org>
---
 fs/ext4/extents_status.c |    8 ++++++++
 fs/ext4/inode.c          |    2 ++
 2 files changed, 10 insertions(+), 0 deletions(-)

diff --git a/fs/ext4/extents_status.c b/fs/ext4/extents_status.c
index 656c82f..71a6e72 100644
--- a/fs/ext4/extents_status.c
+++ b/fs/ext4/extents_status.c
@@ -662,6 +662,14 @@ int ext4_es_insert_extent(struct inode *inode, ext4_lblk_t lblk,
 
 	BUG_ON(end < lblk);
 
+	if ((status & EXTENT_STATUS_DELAYED) &&
+	    (status & EXTENT_STATUS_WRITTEN)) {
+		ext4_warning(inode->i_sb, "Inserting extent [%u/%u] as "
+				" delayed and written which can potentially "
+				" cause data loss.\n", lblk, len);
+		WARN_ON(1);
+	}
+
 	newes.es_lblk = lblk;
 	newes.es_len = len;
 	ext4_es_store_pblock_status(&newes, pblk, status);
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 78fc407..384cf72 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -543,6 +543,7 @@ int ext4_map_blocks(handle_t *handle, struct inode *inode,
 		status = map->m_flags & EXT4_MAP_UNWRITTEN ?
 				EXTENT_STATUS_UNWRITTEN : EXTENT_STATUS_WRITTEN;
 		if (!(flags & EXT4_GET_BLOCKS_DELALLOC_RESERVE) &&
+		    !(status & EXTENT_STATUS_WRITTEN) &&
 		    ext4_find_delalloc_range(inode, map->m_lblk,
 					     map->m_lblk + map->m_len - 1))
 			status |= EXTENT_STATUS_DELAYED;
@@ -657,6 +658,7 @@ found:
 		status = map->m_flags & EXT4_MAP_UNWRITTEN ?
 				EXTENT_STATUS_UNWRITTEN : EXTENT_STATUS_WRITTEN;
 		if (!(flags & EXT4_GET_BLOCKS_DELALLOC_RESERVE) &&
+		    !(status & EXTENT_STATUS_WRITTEN) &&
 		    ext4_find_delalloc_range(inode, map->m_lblk,
 					     map->m_lblk + map->m_len - 1))
 			status |= EXTENT_STATUS_DELAYED;
-- 
1.7.1