Re: System freezes after OOM

[Ondrej Kozina <okozina@redhat.com>]

On 07/14/2016 04:59 PM, Michal Hocko wrote:
> On Thu 14-07-16 10:00:16, Mikulas Patocka wrote:
>>
>>
>> On Thu, 14 Jul 2016, Michal Hocko wrote:
>>
>>> On Wed 13-07-16 11:02:15, Mikulas Patocka wrote:
>>
>>>>> diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
>>>>> index 4f3cb3554944..0b806810efab 100644
>>>>> --- a/drivers/md/dm-crypt.c
>>>>> +++ b/drivers/md/dm-crypt.c
>>>>> @@ -1392,11 +1392,14 @@ static void kcryptd_async_done(struct crypto_async_request *async_req,
>>>>>  static void kcryptd_crypt(struct work_struct *work)
>>>>>  {
>>>>>  	struct dm_crypt_io *io = container_of(work, struct dm_crypt_io, work);
>>>>> +	unsigned int pflags = current->flags;
>>>>>
>>>>> +	current->flags |= PF_LESS_THROTTLE;
>>>>>  	if (bio_data_dir(io->base_bio) == READ)
>>>>>  		kcryptd_crypt_read_convert(io);
>>>>>  	else
>>>>>  		kcryptd_crypt_write_convert(io);
>>>>> +	tsk_restore_flags(current, pflags, PF_LESS_THROTTLE);
>>>>>  }
>>>>>
>>>>>  static void kcryptd_queue_crypt(struct dm_crypt_io *io)
>>>>
>>>> ^^^ That fixes just one specific case - but there may be other threads
>>>> doing mempool allocations in the device mapper subsystem - and you would
>>>> need to mark all of them.
>>>
>>> Now that I am thinking about it some more. Are there any mempool users
>>> which would actually want to be throttled? I would expect mempool users
>>> are necessary to push IO through and throttle them sounds like a bad
>>> decision in the first place but there might be other mempool users which
>>> could cause issues. Anyway how about setting PF_LESS_THROTTLE
>>> unconditionally inside mempool_alloc? Something like the following:
>>>
>>> diff --git a/mm/mempool.c b/mm/mempool.c
>>> index 8f65464da5de..e21fb632983f 100644
>>> --- a/mm/mempool.c
>>> +++ b/mm/mempool.c
>>> @@ -310,7 +310,8 @@ EXPORT_SYMBOL(mempool_resize);
>>>   */
>>>  void *mempool_alloc(mempool_t *pool, gfp_t gfp_mask)
>>>  {
>>> -	void *element;
>>> +	unsigned int pflags = current->flags;
>>> +	void *element = NULL;
>>>  	unsigned long flags;
>>>  	wait_queue_t wait;
>>>  	gfp_t gfp_temp;
>>> @@ -327,6 +328,12 @@ void *mempool_alloc(mempool_t *pool, gfp_t gfp_mask)
>>>
>>>  	gfp_temp = gfp_mask & ~(__GFP_DIRECT_RECLAIM|__GFP_IO);
>>>
>>> +	/*
>>> +	 * Make sure that the allocation doesn't get throttled during the
>>> +	 * reclaim
>>> +	 */
>>> +	if (gfpflags_allow_blocking(gfp_mask))
>>> +		current->flags |= PF_LESS_THROTTLE;
>>>  repeat_alloc:
>>>  	if (likely(pool->curr_nr)) {
>>>  		/*
>>> @@ -339,7 +346,7 @@ void *mempool_alloc(mempool_t *pool, gfp_t gfp_mask)
>>>
>>>  	element = pool->alloc(gfp_temp, pool->pool_data);
>>>  	if (likely(element != NULL))
>>> -		return element;
>>> +		goto out;
>>>
>>>  	spin_lock_irqsave(&pool->lock, flags);
>>>  	if (likely(pool->curr_nr)) {
>>> @@ -352,7 +359,7 @@ void *mempool_alloc(mempool_t *pool, gfp_t gfp_mask)
>>>  		 * for debugging.
>>>  		 */
>>>  		kmemleak_update_trace(element);
>>> -		return element;
>>> +		goto out;
>>>  	}
>>>
>>>  	/*
>>> @@ -369,7 +376,7 @@ void *mempool_alloc(mempool_t *pool, gfp_t gfp_mask)
>>>  	/* We must not sleep if !__GFP_DIRECT_RECLAIM */
>>>  	if (!(gfp_mask & __GFP_DIRECT_RECLAIM)) {
>>>  		spin_unlock_irqrestore(&pool->lock, flags);
>>> -		return NULL;
>>> +		goto out;
>>>  	}
>>>
>>>  	/* Let's wait for someone else to return an element to @pool */
>>> @@ -386,6 +393,10 @@ void *mempool_alloc(mempool_t *pool, gfp_t gfp_mask)
>>>
>>>  	finish_wait(&pool->wait, &wait);
>>>  	goto repeat_alloc;
>>> +out:
>>> +	if (gfpflags_allow_blocking(gfp_mask))
>>> +		tsk_restore_flags(current, pflags, PF_LESS_THROTTLE);
>>> +	return element;
>>>  }
>>>  EXPORT_SYMBOL(mempool_alloc);
>>>
>>
>> But it needs other changes to honor the PF_LESS_THROTTLE flag:
>>
>> static int current_may_throttle(void)
>> {
>>         return !(current->flags & PF_LESS_THROTTLE) ||
>>                 current->backing_dev_info == NULL ||
>>                 bdi_write_congested(current->backing_dev_info);
>> }
>> --- if you set PF_LESS_THROTTLE, current_may_throttle may still return
>> true if one of the other conditions is met.
>
> That is true but doesn't that mean that the device is congested and
> waiting a bit is the right thing to do?
>
>> shrink_zone_memcg calls throttle_vm_writeout without checking
>> PF_LESS_THROTTLE at all.
>
> Yes it doesn't call it because it relies on
> global_dirty_limits()->domain_dirty_limits() to DTRT. It will give the
> caller with PF_LESS_THROTTLE some boost wrt. all other writers.
>

Not sure it'll help but I had to apply following patch to your original 
one. Without it it didn't work.

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index e248194..1616192 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1940,11 +1940,23 @@ bool wb_over_bg_thresh(struct bdi_writeback *wb)
         return false;
  }

+static int current_may_throttle(void)
+{
+       if (current->flags & PF_LESS_THROTTLE)
+               return 0;
+
+       return  current->backing_dev_info == NULL ||
+               bdi_write_congested(current->backing_dev_info);
+}
+
  void throttle_vm_writeout(gfp_t gfp_mask)
  {
         unsigned long background_thresh;
         unsigned long dirty_thresh;

+       if (!current_may_throttle())
+               return;
+
          for ( ; ; ) {
                 global_dirty_limits(&background_thresh, &dirty_thresh);
                 dirty_thresh = hard_dirty_limit(&global_wb_domain, 
dirty_thresh);

Regards Ondra

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