From: Josef Bacik josef@toxicpanda.com

[ Upstream commit 5571e41ec6e56e35f34ae9f5b3a335ef510e0ade ]

While running the CI for an unrelated change I hit the following panic with generic/648 on btrfs_holes_spacecache.

assertion failed: block_start != EXTENT_MAP_HOLE, in fs/btrfs/extent_io.c:1385 ------------[ cut here ]------------ kernel BUG at fs/btrfs/extent_io.c:1385! invalid opcode: 0000 [#1] PREEMPT SMP NOPTI CPU: 1 PID: 2695096 Comm: fsstress Kdump: loaded Tainted: G W 6.8.0-rc2+ #1 RIP: 0010:__extent_writepage_io.constprop.0+0x4c1/0x5c0 Call Trace: <TASK> extent_write_cache_pages+0x2ac/0x8f0 extent_writepages+0x87/0x110 do_writepages+0xd5/0x1f0 filemap_fdatawrite_wbc+0x63/0x90 __filemap_fdatawrite_range+0x5c/0x80 btrfs_fdatawrite_range+0x1f/0x50 btrfs_write_out_cache+0x507/0x560 btrfs_write_dirty_block_groups+0x32a/0x420 commit_cowonly_roots+0x21b/0x290 btrfs_commit_transaction+0x813/0x1360 btrfs_sync_file+0x51a/0x640 __x64_sys_fdatasync+0x52/0x90 do_syscall_64+0x9c/0x190 entry_SYSCALL_64_after_hwframe+0x6e/0x76

This happens because we fail to write out the free space cache in one instance, come back around and attempt to write it again. However on the second pass through we go to call btrfs_get_extent() on the inode to get the extent mapping. Because this is a new block group, and with the free space inode we always search the commit root to avoid deadlocking with the tree, we find nothing and return a EXTENT_MAP_HOLE for the requested range.

This happens because the first time we try to write the space cache out we hit an error, and on an error we drop the extent mapping. This is normal for normal files, but the free space cache inode is special. We always expect the extent map to be correct. Thus the second time through we end up with a bogus extent map.

Since we're deprecating this feature, the most straightforward way to fix this is to simply skip dropping the extent map range for this failed range.

I shortened the test by using error injection to stress the area to make it easier to reproduce. With this patch in place we no longer panic with my error injection test.

CC: stable@vger.kernel.org # 4.14+ Reviewed-by: Filipe Manana fdmanana@suse.com Signed-off-by: Josef Bacik josef@toxicpanda.com Signed-off-by: David Sterba dsterba@suse.com (cherry picked from commit 5571e41ec6e56e35f34ae9f5b3a335ef510e0ade) [Larry: backport to 5.15.y. Minor conflict resolved due to missing commit 4c0c8cfc8433 btrfs: move btrfs_drop_extent_cache() to extent_map.c] Signed-off-by: Larry Bassel larry.bassel@oracle.com --- fs/btrfs/inode.c | 19 +++++++++++++++++-- 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c index d6e43c94436d..e3e5bd4fb477 100644 --- a/fs/btrfs/inode.c +++ b/fs/btrfs/inode.c @@ -3197,8 +3197,23 @@ static int btrfs_finish_ordered_io(struct btrfs_ordered_extent *ordered_extent) unwritten_start += logical_len; clear_extent_uptodate(io_tree, unwritten_start, end, NULL);

- /* Drop the cache for the part of the extent we didn't write. */ - btrfs_drop_extent_cache(inode, unwritten_start, end, 0); + /* + * Drop extent maps for the part of the extent we didn't write. + * + * We have an exception here for the free_space_inode, this is + * because when we do btrfs_get_extent() on the free space inode + * we will search the commit root. If this is a new block group + * we won't find anything, and we will trip over the assert in + * writepage where we do ASSERT(em->block_start != + * EXTENT_MAP_HOLE). + * + * Theoretically we could also skip this for any NOCOW extent as + * we don't mess with the extent map tree in the NOCOW case, but + * for now simply skip this if we are the free space inode. + */ + if (!btrfs_is_free_space_inode(inode)) + btrfs_drop_extent_cache(inode, unwritten_start, + end, 0);

/* * If the ordered extent had an IOERR or something else went