• Filipe Manana's avatar
    Btrfs: fix loss of prealloc extents past i_size after fsync log replay · 471d557a
    Filipe Manana authored
    Currently if we allocate extents beyond an inode's i_size (through the
    fallocate system call) and then fsync the file, we log the extents but
    after a power failure we replay them and then immediately drop them.
    This behaviour happens since about 2009, commit c71bf099 ("Btrfs:
    Avoid orphan inodes cleanup while replaying log"), because it marks
    the inode as an orphan instead of dropping any extents beyond i_size
    before replaying logged extents, so after the log replay, and while
    the mount operation is still ongoing, we find the inode marked as an
    orphan and then perform a truncation (drop extents beyond the inode's
    i_size). Because the processing of orphan inodes is still done
    right after replaying the log and before the mount operation finishes,
    the intention of that commit does not make any sense (at least as
    of today). However reverting that behaviour is not enough, because
    we can not simply discard all extents beyond i_size and then replay
    logged extents, because we risk dropping extents beyond i_size created
    in past transactions, for example:
    
      add prealloc extent beyond i_size
      fsync - clears the flag BTRFS_INODE_NEEDS_FULL_SYNC from the inode
      transaction commit
      add another prealloc extent beyond i_size
      fsync - triggers the fast fsync path
      power failure
    
    In that scenario, we would drop the first extent and then replay the
    second one. To fix this just make sure that all prealloc extents
    beyond i_size are logged, and if we find too many (which is far from
    a common case), fallback to a full transaction commit (like we do when
    logging regular extents in the fast fsync path).
    
    Trivial reproducer:
    
     $ mkfs.btrfs -f /dev/sdb
     $ mount /dev/sdb /mnt
     $ xfs_io -f -c "pwrite -S 0xab 0 256K" /mnt/foo
     $ sync
     $ xfs_io -c "falloc -k 256K 1M" /mnt/foo
     $ xfs_io -c "fsync" /mnt/foo
     <power failure>
    
     # mount to replay log
     $ mount /dev/sdb /mnt
     # at this point the file only has one extent, at offset 0, size 256K
    
    A test case for fstests follows soon, covering multiple scenarios that
    involve adding prealloc extents with previous shrinking truncates and
    without such truncates.
    
    Fixes: c71bf099 ("Btrfs: Avoid orphan inodes cleanup while replaying log")
    Signed-off-by: default avatarFilipe Manana <fdmanana@suse.com>
    Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
    471d557a
tree-log.c 161 KB