From: Boris Burkov boris@bur.io
commit e7db9e5c6b9615b287d01f0231904fbc1fbde9c5 upstream.
We have observed a btrfs filesystem corruption on workloads using no-holes and encoded writes via send stream v2. The symptom is that a file appears to be truncated to the end of its last aligned extent, even though the final unaligned extent and even the file extent and otherwise correctly updated inode item have been written.
So if we were writing out a 1MiB+X file via 8 128K extents and one extent of length X, i_size would be set to 1MiB, but the ninth extent, nbyte, etc. would all appear correct otherwise.
The source of the race is a narrow (one line of code) window in which a no-holes fs has read in an updated i_size, but has not yet set a shared disk_i_size variable to write. Therefore, if two ordered extents run in parallel (par for the course for receive workloads), the following sequence can play out: (following "threads" a bit loosely, since there are callbacks involved for endio but extra threads aren't needed to cause the issue)
ENC-WR1 (second to last) ENC-WR2 (last) ------- ------- btrfs_do_encoded_write set i_size = 1M submit bio B1 ending at 1M endio B1 btrfs_inode_safe_disk_i_size_write local i_size = 1M falls off a cliff for some reason btrfs_do_encoded_write set i_size = 1M+X submit bio B2 ending at 1M+X endio B2 btrfs_inode_safe_disk_i_size_write local i_size = 1M+X disk_i_size = 1M+X disk_i_size = 1M btrfs_delayed_update_inode btrfs_delayed_update_inode
And the delayed inode ends up filled with nbytes=1M+X and isize=1M, and writes respect i_size and present a corrupted file missing its last extents.
Fix this by holding the inode lock in the no-holes case so that a thread can't sneak in a write to disk_i_size that gets overwritten with an out of date i_size.
Fixes: 41a2ee75aab0 ("btrfs: introduce per-inode file extent tree") CC: stable@vger.kernel.org # 5.10+ Reviewed-by: Josef Bacik josef@toxicpanda.com Signed-off-by: Boris Burkov boris@bur.io Reviewed-by: David Sterba dsterba@suse.com Signed-off-by: David Sterba dsterba@suse.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- fs/btrfs/file-item.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-)
--- a/fs/btrfs/file-item.c +++ b/fs/btrfs/file-item.c @@ -47,13 +47,13 @@ void btrfs_inode_safe_disk_i_size_write( u64 start, end, i_size; int ret;
+ spin_lock(&inode->lock); i_size = new_i_size ?: i_size_read(&inode->vfs_inode); if (btrfs_fs_incompat(fs_info, NO_HOLES)) { inode->disk_i_size = i_size; - return; + goto out_unlock; }
- spin_lock(&inode->lock); ret = find_contiguous_extent_bit(&inode->file_extent_tree, 0, &start, &end, EXTENT_DIRTY); if (!ret && start == 0) @@ -61,6 +61,7 @@ void btrfs_inode_safe_disk_i_size_write( else i_size = 0; inode->disk_i_size = i_size; +out_unlock: spin_unlock(&inode->lock); }