[dm-devel] [PATCH v2] dm-bufio: fix deadlock with loop device

[Junxiao Bi]

On 7/10/19 10:48 AM, Mike Snitzer wrote:

> On Tue, Jul 09 2019 at  8:17pm -0400,
> Junxiao Bi <junxiao.bi at oracle.com> wrote:
>
>> When thin-volume was built on loop device, if available memory is low,
>> the following deadlock can be triggered.
>>
>> One process P1 was allocating memory with GFP_FS flag, direct alloc fail,
>> memory reclaim invoked memory shrinker in dm_bufio, dm_bufio_shrink_scan()
>> run, mutex dm_bufio_client->lock was acquired, then P1 wait for dm_buffer
>> io done in __try_evict_buffer->()__try_evict_buffer().
>>
>> But this io may never done as it was issued to the underlying loop device
>> who forward it using fs direct-io, there some memory allocation were using
>> GFP_FS(like do_blockdev_direct_IO()), if direct alloc fail, memory reclaim
>> will invoke memory shrinker in dm_bufio, where dm_bufio_shrink_scan()
>> will be invoked, since the mutex was already hold by P1, the loop thread
>> will hung, io will never done. ABBA deadlock.
>>
>> Signed-off-by: Junxiao Bi <junxiao.bi at oracle.com>
>> ---
>> Changes in v2:
>>    - refine the commit log
>>
>>   drivers/md/dm-bufio.c | 4 +---
>>   1 file changed, 1 insertion(+), 3 deletions(-)
>>
>> diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c
>> index 2a48ea3f1b30..b6b5acc92ca2 100644
>> --- a/drivers/md/dm-bufio.c
>> +++ b/drivers/md/dm-bufio.c
>> @@ -1599,9 +1599,7 @@ dm_bufio_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
>>   	unsigned long freed;
>>   
>>   	c = container_of(shrink, struct dm_bufio_client, shrinker);
>> -	if (sc->gfp_mask & __GFP_FS)
>> -		dm_bufio_lock(c);
>> -	else if (!dm_bufio_trylock(c))
>> +	if (!dm_bufio_trylock(c))
>>   		return SHRINK_STOP;
>>   
>>   	freed  = __scan(c, sc->nr_to_scan, sc->gfp_mask);
>> -- 
>> 2.17.1
>>
> This needs more careful review and understanding.
ok.
>
> I'll commit to getting that done (hopefully with Mikulas' assistance)
> during the 5.3-rcX cycle.  But I'm not ready to stage this change yet.

Hi Mikulas,

Could you help take a look at this?

Thanks,

Junxiao.

>
> Revisiting dm-bufio on loop is needed.  Commit 9d28eb12447ee ("dm bufio:
> change __GFP_IO to __GFP_FS in shrinker callbacks") was meant to address
> deadlocks reported whern running on loop.  And __try_evict_buffer() has
> a check for GFP_NOFS ("!(gfp & __GFP_FS"); but that is only relevant to
> __scan() callers and dm_bufio_shrink_scan() is looking to take the lock
> before __scan() is called.  So it does seem like we have issues in the
> bufio code in general.  Needs a proper audit though.
>
> Mike