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SubjectRe: System freezes after OOM
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.
--
Michal Hocko
SUSE Labs

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