• Rui Hua's avatar
    bcache: recover data from backing when data is clean · e393aa24
    Rui Hua authored
    When we send a read request and hit the clean data in cache device, there
    is a situation called cache read race in bcache(see the commit in the tail
    of cache_look_up(), the following explaination just copy from there):
    The bucket we're reading from might be reused while our bio is in flight,
    and we could then end up reading the wrong data. We guard against this
    by checking (in bch_cache_read_endio()) if the pointer is stale again;
    if so, we treat it as an error (s->iop.error = -EINTR) and reread from
    the backing device (but we don't pass that error up anywhere)
    
    It should be noted that cache read race happened under normal
    circumstances, not the circumstance when SSD failed, it was counted
    and shown in  /sys/fs/bcache/XXX/internal/cache_read_races.
    
    Without this patch, when we use writeback mode, we will never reread from
    the backing device when cache read race happened, until the whole cache
    device is clean, because the condition
    (s->recoverable && (dc && !atomic_read(&dc->has_dirty))) is false in
    cached_dev_read_error(). In this situation, the s->iop.error(= -EINTR)
    will be passed up, at last, user will receive -EINTR when it's bio end,
    this is not suitable, and wield to up-application.
    
    In this patch, we use s->read_dirty_data to judge whether the read
    request hit dirty data in cache device, it is safe to reread data from
    the backing device when the read request hit clean data. This can not
    only handle cache read race, but also recover data when failed read
    request from cache device.
    
    [edited by mlyle to fix up whitespace, commit log title, comment
    spelling]
    
    Fixes: d59b2379 ("bcache: only permit to recovery read error when cache device is clean")
    Cc: <stable@vger.kernel.org> # 4.14
    Signed-off-by: default avatarHua Rui <huarui.dev@gmail.com>
    Reviewed-by: default avatarMichael Lyle <mlyle@lyle.org>
    Reviewed-by: default avatarColy Li <colyli@suse.de>
    Signed-off-by: default avatarMichael Lyle <mlyle@lyle.org>
    Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
    e393aa24
request.c 28.7 KB