Re: System freezes after OOM

[Ondrej Kozina]

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