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SubjectRe: [patch] mm, compaction: ignore watermarks when isolating free pages
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On 06/16/2016 12:34 AM, David Rientjes wrote:
> The goal of memory compaction is to defragment memory by moving migratable
> pages to free pages at the end of the zone. No additional memory is being
> allocated.
>
> Ignore per-zone low watermarks in __isolate_free_page() because memory is
> either fully migrated or isolated free pages are returned when migration
> fails.

Michal Hocko suggested that too, but I didn't think it safe that
compaction should go below the min watermark, even temporarily. It means
the system is struggling with order-0 allocations, so making it worse
for the benefit of high-order allocations doesn't make sense. The
high-order allocation would likely fail anyway due to watermark checks,
even if the page of sufficient order was formed by compaction. So in my
series, I just changed the low watermark check to min [1].

> This fixes an issue where the compaction freeing scanner can isolate
> memory but the zone drops below its low watermark for that page order, so
> the scanner must continue to scan all memory pointlessly.

Good point, looks like failing the watermark is the only reason when
__isolate_free_page() can fail. isolate_freepages_block() and its
callers should take this as an indication that compaction should return
with failure immediately.

[1] http://article.gmane.org/gmane.linux.kernel/2231369

> Signed-off-by: David Rientjes <rientjes@google.com>
> ---
> mm/page_alloc.c | 14 ++------------
> 1 file changed, 2 insertions(+), 12 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2484,23 +2484,14 @@ EXPORT_SYMBOL_GPL(split_page);
>
> int __isolate_free_page(struct page *page, unsigned int order)
> {
> - unsigned long watermark;
> struct zone *zone;
> - int mt;
> + const int mt = get_pageblock_migratetype(page);
>
> BUG_ON(!PageBuddy(page));
> -
> zone = page_zone(page);
> - mt = get_pageblock_migratetype(page);
> -
> - if (!is_migrate_isolate(mt)) {
> - /* Obey watermarks as if the page was being allocated */
> - watermark = low_wmark_pages(zone) + (1 << order);
> - if (!zone_watermark_ok(zone, 0, watermark, 0, 0))
> - return 0;
>
> + if (!is_migrate_isolate(mt))
> __mod_zone_freepage_state(zone, -(1UL << order), mt);
> - }
>
> /* Remove page from free list */
> list_del(&page->lru);
> @@ -2520,7 +2511,6 @@ int __isolate_free_page(struct page *page, unsigned int order)
> }
> }
>
> -
> return 1UL << order;
> }
>
>

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