Btrfs: fix log tree corruption when fs mounted with -o discard

[ Upstream commit HEAD ]

commit dcc82f4783ad91d4ab654f89f37ae9291cdc846a upstream.

While committing a transaction we free the log roots before we write the
new super block. Freeing the log roots implies marking the disk location
of every node/leaf (metadata extent) as pinned before the new super block
is written. This is to prevent the disk location of log metadata extents
from being reused before the new super block is written, otherwise we
would have a corrupted log tree if before the new super block is written
a crash/reboot happens and the location of any log tree metadata extent
ended up being reused and rewritten.

Even though we pinned the log tree's metadata extents, we were issuing a
discard against them if the fs was mounted with the -o discard option,
resulting in corruption of the log tree if a crash/reboot happened before
writing the new super block - the next time the fs was mounted, during
the log replay process we would find nodes/leafs of the log btree with
a content full of zeroes, causing the process to fail and require the
use of the tool btrfs-zero-log to wipeout the log tree (and all data
previously fsynced becoming lost forever).

Fix this by not doing a discard when pinning an extent. The discard will
be done later when it's safe (after the new super block is committed) at
extent-tree.c:btrfs_finish_extent_commit().

Fixes: e688b7252f78 (Btrfs: fix extent pinning bugs in the tree log)
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 3909e5a93ed64a186a396c1b7fd1db07e065728f)
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 4bd5e06fa5ab402f99c5f444059575ac4a3e397f..950479f2d337e52570ce74334b983d9b9926d152 100644 (file)
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -6955,12 +6955,11 @@ static int __btrfs_free_reserved_extent(struct btrfs_root *root,
                return -ENOSPC;
        }
 
-       if (btrfs_test_opt(root, DISCARD))
-               ret = btrfs_discard_extent(root, start, len, NULL);
-
        if (pin)
                pin_down_extent(root, cache, start, len, 1);
        else {
+               if (btrfs_test_opt(root, DISCARD))
+                       ret = btrfs_discard_extent(root, start, len, NULL);
                btrfs_add_free_space(cache, start, len);
                btrfs_update_reserved_bytes(cache, len, RESERVE_FREE, delalloc);
        }