Re: [patch v2] mm, compaction: avoid isolating pinned pages

[Vlastimil Babka]

On 5.2.2014 3:44, David Rientjes wrote:
> Page migration will fail for memory that is pinned in memory with, for
> example, get_user_pages().  In this case, it is unnecessary to take
> zone->lru_lock or isolating the page and passing it to page migration
> which will ultimately fail.
>
> This is a racy check, the page can still change from under us, but in
> that case we'll just fail later when attempting to move the page.
>
> This avoids very expensive memory compaction when faulting transparent
> hugepages after pinning a lot of memory with a Mellanox driver.
>
> On a 128GB machine and pinning ~120GB of memory, before this patch we
> see the enormous disparity in the number of page migration failures
> because of the pinning (from /proc/vmstat):
>
> 	compact_pages_moved 8450
> 	compact_pagemigrate_failed 15614415
>
> 0.05% of pages isolated are successfully migrated and explicitly
> triggering memory compaction takes 102 seconds.  After the patch:
>
> 	compact_pages_moved 9197
> 	compact_pagemigrate_failed 7
>
> 99.9% of pages isolated are now successfully migrated in this
> configuration and memory compaction takes less than one second.
>
> Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
> ---
>   v2: address page count issue per Joonsoo
>
>   mm/compaction.c | 9 +++++++++
>   1 file changed, 9 insertions(+)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -578,6 +578,15 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
>   			continue;
>   		}
>   
> +		/*
> +		 * Migration will fail if an anonymous page is pinned in memory,
> +		 * so avoid taking lru_lock and isolating it unnecessarily in an
> +		 * admittedly racy check.
> +		 */
> +		if (!page_mapping(page) &&
> +		    page_count(page) > page_mapcount(page))
> +			continue;
> +

Hm this page_count() seems it could substantially increase the chance of 
race with prep_compound_page that your patch "mm, page_alloc: make 
first_page visible before PageTail" tries to fix :)

>   		/* Check if it is ok to still hold the lock */
>   		locked = compact_checklock_irqsave(&zone->lru_lock, &flags,
>   								locked, cc);
>
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