• Stefan Hajnoczi's avatar
    qed: do not evict in-use L2 table cache entries · 14fe292d
    Stefan Hajnoczi authored
    The L2 table cache reduces QED metadata reads that would be required
    when translating LBAs to offsets into the image file.  Since requests
    execute in parallel it is possible to share an L2 table between multiple
    requests.
    
    There is a potential data corruption issue when an in-use L2 table is
    evicted from the cache because the following situation occurs:
    
      1. An allocating write performs an update to L2 table "A".
    
      2. Another request needs L2 table "B" and causes table "A" to be
         evicted.
    
      3. A new read request needs L2 table "A" but it is not cached.
    
    As a result the L2 update from #1 can overlap with the L2 fetch from #3.
    We must avoid doing overlapping I/O requests here since the worst case
    outcome is that the L2 fetch completes before the L2 update and yields
    stale data.  In that case we would effectively discard the L2 update and
    lose data clusters!
    
    Thanks to Benoît Canet <benoit.canet@gmail.com> for extensive testing
    and debugging which lead to discovery of this bug.
    Reported-by: default avatarBenoît Canet <benoit.canet@gmail.com>
    Signed-off-by: default avatarStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
    Tested-by: default avatarBenoît Canet <benoit.canet@gmail.com>
    Signed-off-by: default avatarKevin Wolf <kwolf@redhat.com>
    14fe292d
qed-l2-cache.c 5.85 KB