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SubjectRe: System freezes after OOM
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On 07/14/2016 02:51 PM, Michal Hocko wrote:
> On Wed 13-07-16 11:02:15, Mikulas Patocka wrote:
>> On Wed, 13 Jul 2016, Michal Hocko wrote:
> [...]
>
> We are discussing several topics together so let's focus on this
> particlar thing for now
>
>>>> The kernel 4.7-rc almost deadlocks in another way. The machine got stuck
>>>> and the following stacktrace was obtained when swapping to dm-crypt.
>>>>
>>>> We can see that dm-crypt does a mempool allocation. But the mempool
>>>> allocation somehow falls into throttle_vm_writeout. There, it waits for
>>>> 0.1 seconds. So, as a result, the dm-crypt worker thread ends up
>>>> processing requests at an unusually slow rate of 10 requests per second
>>>> and it results in the machine being stuck (it would proabably recover if
>>>> we waited for extreme amount of time).
>>>
>>> Hmm, that throttling is there since ever basically. I do not see what
>>> would have changed that recently, but I haven't looked too close to be
>>> honest.
>>>
>>> I agree that throttling a flusher (which this worker definitely is)
>>> doesn't look like a correct thing to do. We have PF_LESS_THROTTLE for
>>> this kind of things. So maybe the right thing to do is to use this flag
>>> for the dm_crypt worker:
>>>
>>> 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);
>
>

As Mikulas pointed out, this doesn't work. The system froze as well with
the patch above. Will try to tweak the patch with Mikulas's suggestion...

O.

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