Re: [patch -mm 3/3] mm, compaction: avoid compacting memory for thp if pageblock cannot become free

[Vlastimil Babka]

On 06/04/2014 02:30 AM, David Rientjes wrote:
> It's pointless to migrate pages within a pageblock if the entire pageblock will
> not become free for a thp allocation.
>
> If we encounter a page that cannot be migrated and a direct compactor other than
> khugepaged is trying to allocate a hugepage for thp, then skip the entire
> pageblock and avoid migrating pages needlessly.

The problem here is that if you encounter a PageBuddy with order > 0, in 
the next iteration you will see a page that's neither PageBuddy nor 
PageLRU and skip the rest of the pageblock.

I was working on slightly different approach, which is not properly 
tested yet so I will just post a RFC for discussion.

> Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
> ---
>   mm/compaction.c | 41 ++++++++++++++++++++++++++++-------------
>   1 file changed, 28 insertions(+), 13 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -537,12 +537,12 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
>   		if ((low_pfn & (MAX_ORDER_NR_PAGES - 1)) == 0) {
>   			if (!pfn_valid(low_pfn)) {
>   				low_pfn += MAX_ORDER_NR_PAGES - 1;
> -				continue;
> +				goto next;
>   			}
>   		}
>
>   		if (!pfn_valid_within(low_pfn))
> -			continue;
> +			goto next;
>   		nr_scanned++;
>
>   		/*
> @@ -553,7 +553,7 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
>   		 */
>   		page = pfn_to_page(low_pfn);
>   		if (page_zone(page) != zone)
> -			continue;
> +			goto next;
>
>   		if (!valid_page)
>   			valid_page = page;
> @@ -599,7 +599,7 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
>   					goto isolate_success;
>   				}
>   			}
> -			continue;
> +			goto next;
>   		}
>
>   		/*
> @@ -616,7 +616,7 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
>   			if (!locked)
>   				goto next_pageblock;
>   			low_pfn += (1 << compound_order(page)) - 1;
> -			continue;
> +			goto next;
>   		}
>
>   		/*
> @@ -626,7 +626,7 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
>   		 */
>   		if (!page_mapping(page) &&
>   		    page_count(page) > page_mapcount(page))
> -			continue;
> +			goto next;
>
>   		/* Check if it is ok to still hold the lock */
>   		locked = compact_checklock_irqsave(&zone->lru_lock, &flags,
> @@ -636,17 +636,17 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
>
>   		/* Recheck PageLRU and PageTransHuge under lock */
>   		if (!PageLRU(page))
> -			continue;
> +			goto next;
>   		if (PageTransHuge(page)) {
>   			low_pfn += (1 << compound_order(page)) - 1;
> -			continue;
> +			goto next;
>   		}
>
>   		lruvec = mem_cgroup_page_lruvec(page, zone);
>
>   		/* Try isolate the page */
>   		if (__isolate_lru_page(page, mode) != 0)
> -			continue;
> +			goto next;
>
>   		VM_BUG_ON_PAGE(PageTransCompound(page), page);
>
> @@ -669,6 +669,24 @@ isolate_success:
>
>   next_pageblock:
>   		low_pfn = ALIGN(low_pfn + 1, pageblock_nr_pages) - 1;
> +
> +next:
> +		/*
> +		 * It is too expensive for compaction to migrate pages from a
> +		 * pageblock for thp page faults unless the entire pageblock
> +		 * will become free.
> +		 */
> +		if ((cc->gfp_mask & __GFP_NO_KSWAPD) &&
> +		    !(current->flags & PF_KTHREAD)) {
> +			if (locked) {
> +				spin_unlock_irqrestore(&zone->lru_lock, flags);
> +				locked = false;
> +			}
> +			putback_movable_pages(migratelist);
> +			cc->nr_migratepages = 0;
> +			nr_isolated = 0;
> +			low_pfn = ALIGN(low_pfn + 1, pageblock_nr_pages) - 1;
> +		}
>   	}
>
>   	acct_isolated(zone, locked, cc);
> @@ -880,7 +898,7 @@ static isolate_migrate_t isolate_migratepages(struct zone *zone,
>
>   	cc->migrate_pfn = low_pfn;
>
> -	return ISOLATE_SUCCESS;
> +	return cc->nr_migratepages ? ISOLATE_SUCCESS : ISOLATE_NONE;
>   }
>
>   static int compact_finished(struct zone *zone, struct compact_control *cc,
> @@ -1055,9 +1073,6 @@ static int compact_zone(struct zone *zone, struct compact_control *cc)
>   			;
>   		}
>
> -		if (!cc->nr_migratepages)
> -			continue;
> -
>   		err = migrate_pages(&cc->migratepages, compaction_alloc,
>   				compaction_free, (unsigned long)cc, cc->mode,
>   				MR_COMPACTION);

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