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    Btrfs: fix broken free space cache after the system crashed · e570fd27
    Miao Xie 提交于
    When we mounted the filesystem after the crash, we got the following
    message:
      BTRFS error (device xxx): block group xxxx has wrong amount of free space
      BTRFS error (device xxx): failed to load free space cache for block group xxx
    
    It is because we didn't update the metadata of the allocated space (in extent
    tree) until the file data was written into the disk. During this time, there was
    no information about the allocated spaces in either the extent tree nor the
    free space cache. when we wrote out the free space cache at this time (commit
    transaction), those spaces were lost. In fact, only the free space that is
    used to store the file data had this problem, the others didn't because
    the metadata of them is updated in the same transaction context.
    
    There are many methods which can fix the above problem
    - track the allocated space, and write it out when we write out the free
      space cache
    - account the size of the allocated space that is used to store the file
      data, if the size is not zero, don't write out the free space cache.
    
    The first one is complex and may make the performance drop down.
    This patch chose the second method, we use a per-block-group variant to
    account the size of that allocated space. Besides that, we also introduce
    a per-block-group read-write semaphore to avoid the race between
    the allocation and the free space cache write out.
    Signed-off-by: NMiao Xie <miaox@cn.fujitsu.com>
    Signed-off-by: NChris Mason <clm@fb.com>
    e570fd27
extent-tree.c 247.7 KB