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

[Andrew Morton]

On Tue,  5 Apr 2016 13:25:32 +0200 Michal Hocko <mhocko@xxxxxxxxxx> wrote:

> From: Michal Hocko <mhocko@xxxxxxxx>
> 
> 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.