• Mikulas Patocka's avatar
    dm snapshot: avoid snapshot space leak on crash · 20d68d38
    Mikulas Patocka authored
    commit 230c83afdd9cd384348475bea1e14b80b3b6b1b8 upstream.
    
    There is a possible leak of snapshot space in case of crash.
    
    The reason for space leaking is that chunks in the snapshot device are
    allocated sequentially, but they are finished (and stored in the metadata)
    out of order, depending on the order in which copying finished.
    
    For example, supposed that the metadata contains the following records
    SUPERBLOCK
    METADATA (blocks 0 ... 250)
    DATA 0
    DATA 1
    DATA 2
    ...
    DATA 250
    
    Now suppose that you allocate 10 new data blocks 251-260. Suppose that
    copying of these blocks finish out of order (block 260 finished first
    and the block 251 finished last). Now, the snapshot device looks like
    this:
    SUPERBLOCK
    METADATA (blocks 0 ... 250, 260, 259, 258, 257, 256)
    DATA 0
    DATA 1
    DATA 2
    ...
    DATA 250
    DATA 251
    DATA 252
    DATA 253
    DATA 254
    DATA 255
    METADATA (blocks 255, 254, 253, 252, 251)
    DATA 256
    DATA 257
    DATA 258
    DATA 259
    DATA 260
    
    Now, if the machine crashes after writing the first metadata block but
    before writing the second metadata block, the space for areas DATA 250-255
    is leaked, it contains no valid data and it will never be used in the
    future.
    
    This patch makes dm-snapshot complete exceptions in the same order they
    were allocated, thus fixing this bug.
    
    Note: when backporting this patch to the stable kernel, change the version
    field in the following way:
    * if version in the stable kernel is {1, 11, 1}, change it to {1, 12, 0}
    * if version in the stable kernel is {1, 10, 0} or {1, 10, 1}, change it
      to {1, 10, 2}
    Userspace reads the version to determine if the bug was fixed, so the
    version change is needed.
    Signed-off-by: default avatarMikulas Patocka <mpatocka@redhat.com>
    Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    20d68d38
dm-snap.c 54.1 KB