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

[Joonsoo Kim]

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.