On Tue 05-04-16 17:06:12, Andrew Morton wrote: > 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. 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>