Re: [PATCH 10/11] mm, oom: protect !costly allocations some more

[Michal Hocko <mhocko@kernel.org>]

On Tue 05-04-16 17:06:12, Andrew Morton wrote:
> On Tue,  5 Apr 2016 13:25:32 +0200 Michal Hocko <mhocko@kernel.org> wrote:
> 
> > From: Michal Hocko <mhocko@suse.com>
> > 
> > should_reclaim_retry will give up retries for higher order allocations
> > if none of the eligible zones has any requested or higher order pages
> > available even if we pass the watermak check for order-0. This is done
> > because there is no guarantee that the reclaimable and currently free
> > pages will form the required order.
> > 
> > This can, however, lead to situations were the high-order request (e.g.
> > order-2 required for the stack allocation during fork) will trigger
> > OOM too early - e.g. after the first reclaim/compaction round. Such a
> > system would have to be highly fragmented and there is no guarantee
> > further reclaim/compaction attempts would help but at least make sure
> > that the compaction was active before we go OOM and keep retrying even
> > if should_reclaim_retry tells us to oom if
> > 	- the last compaction round backed off or
> > 	- we haven't completed at least MAX_COMPACT_RETRIES active
> > 	  compaction rounds.
> > 
> > The first rule ensures that the very last attempt for compaction
> > was not ignored while the second guarantees that the compaction has done
> > some work. Multiple retries might be needed to prevent occasional
> > pigggy packing of other contexts to steal the compacted pages before
> > the current context manages to retry to allocate them.
> > 
> > compaction_failed() is taken as a final word from the compaction that
> > the retry doesn't make much sense. We have to be careful though because
> > the first compaction round is MIGRATE_ASYNC which is rather weak as it
> > ignores pages under writeback and gives up too easily in other
> > situations. We therefore have to make sure that MIGRATE_SYNC_LIGHT mode
> > has been used before we give up. With this logic in place we do not have
> > to increase the migration mode unconditionally and rather do it only if
> > the compaction failed for the weaker mode. A nice side effect is that
> > the stronger migration mode is used only when really needed so this has
> > a potential of smaller latencies in some cases.
> > 
> > Please note that the compaction doesn't tell us much about how
> > successful it was when returning compaction_made_progress so we just
> > have to blindly trust that another retry is worthwhile and cap the
> > number to something reasonable to guarantee a convergence.
> > 
> > If the given number of successful retries is not sufficient for a
> > reasonable workloads we should focus on the collected compaction
> > tracepoints data and try to address the issue in the compaction code.
> > If this is not feasible we can increase the retries limit.
> > 
> > @@ -3369,14 +3425,6 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> >  	if (is_thp_gfp_mask(gfp_mask) && compaction_withdrawn(compact_result))
> >  		goto nopage;
> >  
> > -	/*
> > -	 * It can become very expensive to allocate transparent hugepages at
> > -	 * fault, so use asynchronous memory compaction for THP unless it is
> > -	 * khugepaged trying to collapse.
> > -	 */
> > -	if (!is_thp_gfp_mask(gfp_mask) || (current->flags & PF_KTHREAD))
> > -		migration_mode = MIGRATE_SYNC_LIGHT;
> > -
> >  	/* Try direct reclaim and then allocating */
> >  	page = __alloc_pages_direct_reclaim(gfp_mask, order, alloc_flags, ac,
> >  							&did_some_progress);
> 
> Hugh's patches moved this elsewhere.  I'll drop this hunk altogether -
> please carefully review the result.

I have checked mm-oom-protect-costly-allocations-some-more.patch and it
kept the hunk which is the correct way to go because migration_mode
should be updated only in should_compact_retry or before the last
attempt for __alloc_pages_direct_compact before we fail (for
!__GFP_REPEAT resp. __GFP_NORETRY).

Thanks!
-- 
Michal Hocko
SUSE Labs

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