Re: dm bufio: fix deadlock issue with loop device

[Mike Snitzer <snitzer@redhat.com>]

On Tue, Jul 02 2019 at  7:14pm -0400,
Junxiao Bi <junxiao.bi@oracle.com> wrote:

> The following deadlock was caputred on 4.1, since dm_bufio_shrink_count
> still had bufio lock acquired, this was already fixed by mainline. But
> shrinker will also invoke dm_bufio_shrink_scan by ->scan_objects, so
> looks like mainline will suffer the same deadlock issue.
> 
> This deadlock happened when both kswapd0 and loop1 were shrinking memory,
> kswapd0 hold bufio lock and waiting for an in-flight io done, but it will
> never done because loop1 who was issuing the io was hung by the same lock
> hold by kswapd0. This was ABBA deadlock.
> 
> The gfp_flags used in direct IO is GFP_KERNEL, so checking GFP_FS/IO won't
> work, just stop shrinking if lock was hold by others.
> 
> PID: 474    TASK: ffff8813e11f4600  CPU: 10  COMMAND: "kswapd0"
>    #0 [ffff8813dedfb938] __schedule at ffffffff8173f405
>    #1 [ffff8813dedfb990] schedule at ffffffff8173fa27
>    #2 [ffff8813dedfb9b0] schedule_timeout at ffffffff81742fec
>    #3 [ffff8813dedfba60] io_schedule_timeout at ffffffff8173f186
>    #4 [ffff8813dedfbaa0] bit_wait_io at ffffffff8174034f
>    #5 [ffff8813dedfbac0] __wait_on_bit at ffffffff8173fec8
>    #6 [ffff8813dedfbb10] out_of_line_wait_on_bit at ffffffff8173ff81
>    #7 [ffff8813dedfbb90] __make_buffer_clean at ffffffffa038736f [dm_bufio]
>    #8 [ffff8813dedfbbb0] __try_evict_buffer at ffffffffa0387bb8 [dm_bufio]
>    #9 [ffff8813dedfbbd0] dm_bufio_shrink_scan at ffffffffa0387cc3 [dm_bufio]
>   #10 [ffff8813dedfbc40] shrink_slab at ffffffff811a87ce
>   #11 [ffff8813dedfbd30] shrink_zone at ffffffff811ad778
>   #12 [ffff8813dedfbdc0] kswapd at ffffffff811ae92f
>   #13 [ffff8813dedfbec0] kthread at ffffffff810a8428
>   #14 [ffff8813dedfbf50] ret_from_fork at ffffffff81745242
> 
>   PID: 14127  TASK: ffff881455749c00  CPU: 11  COMMAND: "loop1"
>    #0 [ffff88272f5af228] __schedule at ffffffff8173f405
>    #1 [ffff88272f5af280] schedule at ffffffff8173fa27
>    #2 [ffff88272f5af2a0] schedule_preempt_disabled at ffffffff8173fd5e
>    #3 [ffff88272f5af2b0] __mutex_lock_slowpath at ffffffff81741fb5
>    #4 [ffff88272f5af330] mutex_lock at ffffffff81742133
>    #5 [ffff88272f5af350] dm_bufio_shrink_count at ffffffffa03865f9 [dm_bufio]
>    #6 [ffff88272f5af380] shrink_slab at ffffffff811a86bd
>    #7 [ffff88272f5af470] shrink_zone at ffffffff811ad778
>    #8 [ffff88272f5af500] do_try_to_free_pages at ffffffff811adb34
>    #9 [ffff88272f5af590] try_to_free_pages at ffffffff811adef8
>   #10 [ffff88272f5af610] __alloc_pages_nodemask at ffffffff811a09c3
>   #11 [ffff88272f5af710] alloc_pages_current at ffffffff811e8b71
>   #12 [ffff88272f5af760] new_slab at ffffffff811f4523
>   #13 [ffff88272f5af7b0] __slab_alloc at ffffffff8173a1b5
>   #14 [ffff88272f5af880] kmem_cache_alloc at ffffffff811f484b
>   #15 [ffff88272f5af8d0] do_blockdev_direct_IO at ffffffff812535b3
>   #16 [ffff88272f5afb00] __blockdev_direct_IO at ffffffff81255dc3
>   #17 [ffff88272f5afb30] xfs_vm_direct_IO at ffffffffa01fe3fc [xfs]
>   #18 [ffff88272f5afb90] generic_file_read_iter at ffffffff81198994
>   #19 [ffff88272f5afc50] __dta_xfs_file_read_iter_2398 at ffffffffa020c970 [xfs]
>   #20 [ffff88272f5afcc0] lo_rw_aio at ffffffffa0377042 [loop]
>   #21 [ffff88272f5afd70] loop_queue_work at ffffffffa0377c3b [loop]
>   #22 [ffff88272f5afe60] kthread_worker_fn at ffffffff810a8a0c
>   #23 [ffff88272f5afec0] kthread at ffffffff810a8428
>   #24 [ffff88272f5aff50] ret_from_fork at ffffffff81745242
> 
> Signed-off-by: Junxiao Bi <junxiao.bi@oracle.com>
> ---
>  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
> 

I don't follow how this fixes the direct IO to DM device ontop of loop
case given that you're saying __GFP_FS will not have been set by the
direct IO path.  In that case it should resort to the trylock anyway,
no?

We need a reproducer in the context of the latest upstream kernel code,
not some 4.1 branch point for an Oracle kernel.

Please submit with a less conflated patch header that has a description
of the bufio issue that the upstream kernel has.

Thanks,
Mike