• Qu Wenruo's avatar
    btrfs: scrub: Don't check free space before marking a block group RO · b12de528
    Qu Wenruo authored
    
    
    [BUG]
    When running btrfs/072 with only one online CPU, it has a pretty high
    chance to fail:
    
      btrfs/072 12s ... _check_dmesg: something found in dmesg (see xfstests-dev/results//btrfs/072.dmesg)
      - output mismatch (see xfstests-dev/results//btrfs/072.out.bad)
          --- tests/btrfs/072.out     2019-10-22 15:18:14.008965340 +0800
          +++ /xfstests-dev/results//btrfs/072.out.bad      2019-11-14 15:56:45.877152240 +0800
          @@ -1,2 +1,3 @@
           QA output created by 072
           Silence is golden
          +Scrub find errors in "-m dup -d single" test
          ...
    
    And with the following call trace:
    
      BTRFS info (device dm-5): scrub: started on devid 1
      ------------[ cut here ]------------
      BTRFS: Transaction aborted (error -27)
      WARNING: CPU: 0 PID: 55087 at fs/btrfs/block-group.c:1890 btrfs_create_pending_block_groups+0x3e6/0x470 [btrfs]
      CPU: 0 PID: 55087 Comm: btrfs Tainted: G        W  O      5.4.0-rc1-custom+ #13
      Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
      RIP: 0010:btrfs_create_pending_block_groups+0x3e6/0x470 [btrfs]
      Call Trace:
       __btrfs_end_transaction+0xdb/0x310 [btrfs]
       btrfs_end_transaction+0x10/0x20 [btrfs]
       btrfs_inc_block_group_ro+0x1c9/0x210 [btrfs]
       scrub_enumerate_chunks+0x264/0x940 [btrfs]
       btrfs_scrub_dev+0x45c/0x8f0 [btrfs]
       btrfs_ioctl+0x31a1/0x3fb0 [btrfs]
       do_vfs_ioctl+0x636/0xaa0
       ksys_ioctl+0x67/0x90
       __x64_sys_ioctl+0x43/0x50
       do_syscall_64+0x79/0xe0
       entry_SYSCALL_64_after_hwframe+0x49/0xbe
      ---[ end trace 166c865cec7688e7 ]---
    
    [CAUSE]
    The error number -27 is -EFBIG, returned from the following call chain:
    btrfs_end_transaction()
    |- __btrfs_end_transaction()
       |- btrfs_create_pending_block_groups()
          |- btrfs_finish_chunk_alloc()
             |- btrfs_add_system_chunk()
    
    This happens because we have used up all space of
    btrfs_super_block::sys_chunk_array.
    
    The root cause is, we have the following bad loop of creating tons of
    system chunks:
    
    1. The only SYSTEM chunk is being scrubbed
       It's very common to have only one SYSTEM chunk.
    2. New SYSTEM bg will be allocated
       As btrfs_inc_block_group_ro() will check if we have enough space
       after marking current bg RO. If not, then allocate a new chunk.
    3. New SYSTEM bg is still empty, will be reclaimed
       During the reclaim, we will mark it RO again.
    4. That newly allocated empty SYSTEM bg get scrubbed
       We go back to step 2, as the bg is already mark RO but still not
       cleaned up yet.
    
    If the cleaner kthread doesn't get executed fast enough (e.g. only one
    CPU), then we will get more and more empty SYSTEM chunks, using up all
    the space of btrfs_super_block::sys_chunk_array.
    
    [FIX]
    Since scrub/dev-replace doesn't always need to allocate new extent,
    especially chunk tree extent, so we don't really need to do chunk
    pre-allocation.
    
    To break above spiral, here we introduce a new parameter to
    btrfs_inc_block_group(), @do_chunk_alloc, which indicates whether we
    need extra chunk pre-allocation.
    
    For relocation, we pass @do_chunk_alloc=true, while for scrub, we pass
    @do_chunk_alloc=false.
    This should keep unnecessary empty chunks from popping up for scrub.
    
    Also, since there are two parameters for btrfs_inc_block_group_ro(),
    add more comment for it.
    
    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>
    b12de528