Re: [PATCH] dm: Avoid sleeping while holding the dm_bufio lock

[Doug Anderson]

Hi,

On Wed, Nov 23, 2016 at 12:57 PM, Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote:
> Hi
>
> The GFP_NOIO allocation frees clean cached pages. The GFP_NOWAIT
> allocation doesn't. Your patch would incorrectly reuse buffers in a
> situation when the memory is filled with clean cached pages.
>
> Here I'm proposing an alternate patch that first tries GFP_NOWAIT
> allocation, then drops the lock and tries GFP_NOIO allocation.
>
> Note that the root cause why you are seeing this stacktrace is, that your
> block device is congested - i.e. there are too many requests in the
> device's queue - and note that fixing this wait won't fix the root cause
> (congested device).
>
> The congestion limits are set in blk_queue_congestion_threshold to 7/8 to
> 13/16 size of the nr_requests value.
>
> If you don't want your device to report the congested status, you can
> increase /sys/block/<device>/queue/nr_requests - you should test if your
> chromebook is faster of slower with this setting increased. But note that
> this setting won't increase the IO-per-second of the device.

Cool, thanks for the insight!

Can you clarify which block device is relevant here?  Is this the DM
block device, the underlying block device, or the swap block device?
I'm not at all an expert on DM, but I think we have:

1. /dev/mmcblk0 - the underlying storage device.
2. /dev/dm-0 - The verity device that's run atop /dev/mmcblk0p3
3. /dev/zram0 - Our swap device

As stated in the original email, I'm running on a downstream kernel
(kernel-4.4) with bunches of local patches, so it's plausible that
things have changed in the meantime, but:

* At boot time the "nr_requests" for all block devices was 128
* I was unable to set the "nr_requests" for dm-0 and zram0 (it just
gives an error in sysfs).
* When I set "nr_requests" to 4096 for /dev/mmcblk0 it didn't seem to
affect the problem.


> Mikulas
>
>
> On Thu, 17 Nov 2016, Douglas Anderson wrote:
>
>> We've seen in-field reports showing _lots_ (18 in one case, 41 in
>> another) of tasks all sitting there blocked on:
>>
>>   mutex_lock+0x4c/0x68
>>   dm_bufio_shrink_count+0x38/0x78
>>   shrink_slab.part.54.constprop.65+0x100/0x464
>>   shrink_zone+0xa8/0x198
>>
>> In the two cases analyzed, we see one task that looks like this:
>>
>>   Workqueue: kverityd verity_prefetch_io
>>
>>   __switch_to+0x9c/0xa8
>>   __schedule+0x440/0x6d8
>>   schedule+0x94/0xb4
>>   schedule_timeout+0x204/0x27c
>>   schedule_timeout_uninterruptible+0x44/0x50
>>   wait_iff_congested+0x9c/0x1f0
>>   shrink_inactive_list+0x3a0/0x4cc
>>   shrink_lruvec+0x418/0x5cc
>>   shrink_zone+0x88/0x198
>>   try_to_free_pages+0x51c/0x588
>>   __alloc_pages_nodemask+0x648/0xa88
>>   __get_free_pages+0x34/0x7c
>>   alloc_buffer+0xa4/0x144
>>   __bufio_new+0x84/0x278
>>   dm_bufio_prefetch+0x9c/0x154
>>   verity_prefetch_io+0xe8/0x10c
>>   process_one_work+0x240/0x424
>>   worker_thread+0x2fc/0x424
>>   kthread+0x10c/0x114
>>
>> ...and that looks to be the one holding the mutex.
>>
>> The problem has been reproduced on fairly easily:
>> 0. Be running Chrome OS w/ verity enabled on the root filesystem
>> 1. Pick test patch: http://crosreview.com/412360
>> 2. Install launchBalloons.sh and balloon.arm from
>>      http://crbug.com/468342
>>    ...that's just a memory stress test app.
>> 3. On a 4GB rk3399 machine, run
>>      nice ./launchBalloons.sh 4 900 100000
>>    ...that tries to eat 4 * 900 MB of memory and keep accessing.
>> 4. Login to the Chrome web browser and restore many tabs
>>
>> With that, I've seen printouts like:
>>   DOUG: long bufio 90758 ms
>> ...and stack trace always show's we're in dm_bufio_prefetch().
>>
>> The problem is that we try to allocate memory with GFP_NOIO while
>> we're holding the dm_bufio lock.  Instead we should be using
>> GFP_NOWAIT.  Using GFP_NOIO can cause us to sleep while holding the
>> lock and that causes the above problems.
>>
>> The current behavior explained by David Rientjes:
>>
>>   It will still try reclaim initially because __GFP_WAIT (or
>>   __GFP_KSWAPD_RECLAIM) is set by GFP_NOIO.  This is the cause of
>>   contention on dm_bufio_lock() that the thread holds.  You want to
>>   pass GFP_NOWAIT instead of GFP_NOIO to alloc_buffer() when holding a
>>   mutex that can be contended by a concurrent slab shrinker (if
>>   count_objects didn't use a trylock, this pattern would trivially
>>   deadlock).
>>
>> Suggested-by: David Rientjes <rientjes@xxxxxxxxxx>
>> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
>> ---
>> Note that this change was developed and tested against the Chrome OS
>> 4.4 kernel tree, not mainline.  Due to slight differences in verity
>> between mainline and Chrome OS it became too difficult to reproduce my
>> testing setup on mainline.  This patch still seems correct and
>> relevant to upstream, so I'm posting it.  If this is not acceptible to
>> you then please ignore this patch.
>>
>> Also note that when I tested the Chrome OS 3.14 kernel tree I couldn't
>> reproduce the long delays described in the patch.  Presumably
>> something changed in either the kernel config or the memory management
>> code between the two kernel versions that made this crop up.  In a
>> similar vein, it is possible that problems described in this patch are
>> no longer reproducible upstream.  However, the arguments made in this
>> patch (that we don't want to block while holding the mutex) still
>> apply so I think the patch may still have merit.
>>
>>  drivers/md/dm-bufio.c | 6 ++++--
>>  1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c
>> index b3ba142e59a4..3c767399cc59 100644
>> --- a/drivers/md/dm-bufio.c
>> +++ b/drivers/md/dm-bufio.c
>> @@ -827,7 +827,8 @@ static struct dm_buffer *__alloc_buffer_wait_no_callback(struct dm_bufio_client
>>        * dm-bufio is resistant to allocation failures (it just keeps
>>        * one buffer reserved in cases all the allocations fail).
>>        * So set flags to not try too hard:
>> -      *      GFP_NOIO: don't recurse into the I/O layer
>> +      *      GFP_NOWAIT: don't wait; if we need to sleep we'll release our
>> +      *                  mutex and wait ourselves.
>>        *      __GFP_NORETRY: don't retry and rather return failure
>>        *      __GFP_NOMEMALLOC: don't use emergency reserves
>>        *      __GFP_NOWARN: don't print a warning in case of failure
>> @@ -837,7 +838,8 @@ static struct dm_buffer *__alloc_buffer_wait_no_callback(struct dm_bufio_client
>>        */
>>       while (1) {
>>               if (dm_bufio_cache_size_latch != 1) {
>> -                     b = alloc_buffer(c, GFP_NOIO | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN);
>> +                     b = alloc_buffer(c, GFP_NOWAIT | __GFP_NORETRY |
>> +                                      __GFP_NOMEMALLOC | __GFP_NOWARN);
>>                       if (b)
>>                               return b;
>>               }
>> --
>> 2.8.0.rc3.226.g39d4020
>>
>> --
>> dm-devel mailing list
>> dm-devel@xxxxxxxxxx
>> https://www.redhat.com/mailman/listinfo/dm-devel
>>
>
> From: Mikulas Patocka <mpatocka@xxxxxxxxxx>
>
> Subject: dm-bufio: drop the lock when doing GFP_NOIO alloaction
>
> Drop the lock when doing GFP_NOIO alloaction beacuse the allocation can
> take some time.
>
> Note that we won't do GFP_NOIO allocation when we loop for the second
> time, because the lock shouldn't be dropped between __wait_for_free_buffer
> and __get_unclaimed_buffer.
>
> Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
>
> ---
>  drivers/md/dm-bufio.c |   13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
>
> Index: linux-2.6/drivers/md/dm-bufio.c
> ===================================================================
> --- linux-2.6.orig/drivers/md/dm-bufio.c
> +++ linux-2.6/drivers/md/dm-bufio.c
> @@ -822,11 +822,13 @@ enum new_flag {
>  static struct dm_buffer *__alloc_buffer_wait_no_callback(struct dm_bufio_client *c, enum new_flag nf)
>  {
>         struct dm_buffer *b;
> +       bool tried_noio_alloc = false;
>
>         /*
>          * dm-bufio is resistant to allocation failures (it just keeps
>          * one buffer reserved in cases all the allocations fail).
>          * So set flags to not try too hard:
> +        *      GFP_NOWAIT: don't sleep and don't release cache
>          *      GFP_NOIO: don't recurse into the I/O layer
>          *      __GFP_NORETRY: don't retry and rather return failure
>          *      __GFP_NOMEMALLOC: don't use emergency reserves
> @@ -837,7 +839,7 @@ static struct dm_buffer *__alloc_buffer_
>          */
>         while (1) {
>                 if (dm_bufio_cache_size_latch != 1) {
> -                       b = alloc_buffer(c, GFP_NOIO | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN);
> +                       b = alloc_buffer(c, GFP_NOWAIT | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN);
>                         if (b)
>                                 return b;
>                 }
> @@ -845,6 +847,15 @@ static struct dm_buffer *__alloc_buffer_
>                 if (nf == NF_PREFETCH)
>                         return NULL;
>
> +               if (dm_bufio_cache_size_latch != 1 && !tried_noio_alloc) {
> +                       dm_bufio_unlock(c);
> +                       b = alloc_buffer(c, GFP_NOIO | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN);
> +                       dm_bufio_lock(c);
> +                       if (b)
> +                               return b;
> +                       tried_noio_alloc = true;
> +               }
> +
>                 if (!list_empty(&c->reserved_buffers)) {
>                         b = list_entry(c->reserved_buffers.next,
>                                        struct dm_buffer, lru_list);

I agree that I believe that it is safe to drop and re-grab the
dm_bufio lock in this function.  I also believe it to be safe to call
alloc_buffer() without holding the dm_bufio lock.

That means that this looks fine to me.  It also fixes the test case
that I have.  ...so for what it's worth:

Reviewed-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
Tested-by: Douglas Anderson <dianders@xxxxxxxxxxxx>

-Doug