Btrfs: deal with duplciates during extent_map insertion in btrfs_get_extent

Orabug: 27446668

When dealing with inline extents, btrfs_get_extent will incorrectly try
to insert a duplicate extent_map.  The dup hits -EEXIST from
add_extent_map, but then we try to merge with the existing one and end
up trying to insert a zero length extent_map.

This actually works most of the time, except when there are extent maps
past the end of the inline extent.  rocksdb will trigger this sometimes
because it preallocates an extent and then truncates down.

Josef made a script to trigger with xfs_io:

	#!/bin/bash

	xfs_io -f -c "pwrite 0 1000" inline
	xfs_io -c "falloc -k 4k 1M" inline
	xfs_io -c "pread 0 1000" -c "fadvise -d 0 1000" -c "pread 0 1000" inline
	xfs_io -c "fadvise -d 0 1000" inline
	cat inline

You'll get EIOs trying to read inline after this because add_extent_map
is returning EEXIST

Signed-off-by: Chris Mason <clm@fb.com>
(cherry picked from commit 8dff9c85341032767d7b519217a79ea04cd676b0)
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 37a43c15d92189a09689bc5d30ea7ff2c2d8e332..c331e03e581c9ca56bb3afda10142598e34cb906 100644 (file)
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -6945,7 +6945,18 @@ insert:
                 * existing will always be non-NULL, since there must be
                 * extent causing the -EEXIST.
                 */
-               if (start >= extent_map_end(existing) ||
+               if (existing->start == em->start &&
+                   extent_map_end(existing) == extent_map_end(em) &&
+                   em->block_start == existing->block_start) {
+                       /*
+                        * these two extents are the same, it happens
+                        * with inlines especially
+                        */
+                       free_extent_map(em);
+                       em = existing;
+                       err = 0;
+
+               } else if (start >= extent_map_end(existing) ||
                    start <= existing->start) {
                        /*
                         * The existing extent map is the one nearest to