Re: [RFC 12/13] mm, compaction: more reliably increase direct compaction priority

Joonsoo Kim
2016-06-02T02:49:34Z

On Tue, May 31, 2016 at 02:29:24PM +0200, Vlastimil Babka wrote:
> On 05/31/2016 02:07 PM, Vlastimil Babka wrote:
> >On 05/31/2016 08:37 AM, Joonsoo Kim wrote:
> >>>@@ -3695,22 +3695,22 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> >>> else
> >>> no_progress_loops++;
> >>>
> >>>- if (should_reclaim_retry(gfp_mask, order, ac, alloc_flags,
> >>>- did_some_progress > 0, no_progress_loops))
> >>>- goto retry;
> >>>-
> >>>+ should_retry = should_reclaim_retry(gfp_mask, order, ac, alloc_flags,
> >>>+ did_some_progress > 0, no_progress_loops);
> >>> /*
> >>> * It doesn't make any sense to retry for the compaction if the order-0
> >>> * reclaim is not able to make any progress because the current
> >>> * implementation of the compaction depends on the sufficient amount
> >>> * of free memory (see __compaction_suitable)
> >>> */
> >>>- if (did_some_progress > 0 &&
> >>>- should_compact_retry(ac, order, alloc_flags,
> >>>+ if (did_some_progress > 0)
> >>>+ should_retry |= should_compact_retry(ac, order, alloc_flags,
> >>> compact_result, &compact_priority,
> >>>- compaction_retries))
> >>>+ compaction_retries);
> >>>+ if (should_retry)
> >>> goto retry;
> >>
> >>Hmm... it looks odd that we check should_compact_retry() when
> >>did_some_progress > 0. If system is full of anonymous memory and we
> >>don't have swap, we can't reclaim anything but we can compact.
> >
> >Right, thanks.
>
> Hmm on the other hand, should_compact_retry will assume (in
> compaction_zonelist_suitable()) that reclaimable memory is actually
> reclaimable. If there's nothing to tell us that it actually isn't,
> if we drop the reclaim progress requirement. That's risking an
> infinite loop?

You are right. I hope this retry logic will be robust to cover all
the theoretical situations but it looks not easy. Sigh...

Thanks.