• Qu Wenruo's avatar
    btrfs: scrub: Require mandatory block group RO for dev-replace · 1bbb97b8
    Qu Wenruo authored
    [BUG]
    For dev-replace test cases with fsstress, like btrfs/06[45] btrfs/071,
    looped runs can lead to random failure, where scrub finds csum error.
    
    The possibility is not high, around 1/20 to 1/100, but it's causing data
    corruption.
    
    The bug is observable after commit b12de528 ("btrfs: scrub: Don't
    check free space before marking a block group RO")
    
    [CAUSE]
    Dev-replace has two source of writes:
    
    - Write duplication
      All writes to source device will also be duplicated to target device.
    
      Content:	Not yet persisted data/meta
    
    - Scrub copy
      Dev-replace reused scrub code to iterate through existing extents, and
      copy the verified data to target device.
    
      Content:	Previously persisted data and metadata
    
    The difference in contents makes the following race possible:
    	Regular Writer		|	Dev-replace
    -----------------------------------------------------------------
      ^                             |
      | Preallocate one data extent |
      | at bytenr X, len 1M		|
      v				|
      ^ Commit transaction		|
      | Now extent [X, X+1M) is in  |
      v commit root			|
     ================== Dev replace starts =========================
      				| ^
    				| | Scrub extent [X, X+1M)
    				| | Read [X, X+1M)
    				| | (The content are mostly garbage
    				| |  since it's preallocated)
      ^				| v
      | Write back happens for	|
      | extent [X, X+512K)		|
      | New data writes to both	|
      | source and target dev.	|
      v				|
    				| ^
    				| | Scrub writes back extent [X, X+1M)
    				| | to target device.
    				| | This will over write the new data in
    				| | [X, X+512K)
    				| v
    
    This race can only happen for nocow writes. Thus metadata and data cow
    writes are safe, as COW will never overwrite extents of previous
    transaction (in commit root).
    
    This behavior can be confirmed by disabling all fallocate related calls
    in fsstress (*), then all related tests can pass a 2000 run loop.
    
    *: FSSTRESS_AVOID="-f fallocate=0 -f allocsp=0 -f zero=0 -f insert=0 \
    		   -f collapse=0 -f punch=0 -f resvsp=0"
       I didn't expect resvsp ioctl will fallback to fallocate in VFS...
    
    [FIX]
    Make dev-replace to require mandatory block group RO, and wait for current
    nocow writes before calling scrub_chunk().
    
    This patch will mostly revert commit 76a8efa1
    
     ("btrfs: Continue replace
    when set_block_ro failed") for dev-replace path.
    
    The side effect is, dev-replace can be more strict on avaialble space, but
    definitely worth to avoid data corruption.
    Reported-by: default avatarFilipe Manana <fdmanana@suse.com>
    Fixes: 76a8efa1 ("btrfs: Continue replace when set_block_ro failed")
    Fixes: b12de528
    
     ("btrfs: scrub: Don't check free space before marking a block group RO")
    Reviewed-by: default avatarFilipe Manana <fdmanana@suse.com>
    Signed-off-by: default avatarQu Wenruo <wqu@suse.com>
    Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
    1bbb97b8