Re: [PATCH v3 09/13] mm, compaction: skip buddy pages by their order in the migrate scanner

[Zhang Yanfei <zhangyanfei@cn.fujitsu.com>]

On 06/20/2014 11:49 PM, Vlastimil Babka wrote:
> The migration scanner skips PageBuddy pages, but does not consider their order
> as checking page_order() is generally unsafe without holding the zone->lock,
> and acquiring the lock just for the check wouldn't be a good tradeoff.
> 
> Still, this could avoid some iterations over the rest of the buddy page, and
> if we are careful, the race window between PageBuddy() check and page_order()
> is small, and the worst thing that can happen is that we skip too much and miss
> some isolation candidates. This is not that bad, as compaction can already fail
> for many other reasons like parallel allocations, and those have much larger
> race window.
> 
> This patch therefore makes the migration scanner obtain the buddy page order
> and use it to skip the whole buddy page, if the order appears to be in the
> valid range.
> 
> It's important that the page_order() is read only once, so that the value used
> in the checks and in the pfn calculation is the same. But in theory the
> compiler can replace the local variable by multiple inlines of page_order().
> Therefore, the patch introduces page_order_unsafe() that uses ACCESS_ONCE to
> prevent this.
> 
> Testing with stress-highalloc from mmtests shows a 15% reduction in number of
> pages scanned by migration scanner. This change is also a prerequisite for a
> later patch which is detecting when a cc->order block of pages contains
> non-buddy pages that cannot be isolated, and the scanner should thus skip to
> the next block immediately.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Cc: Michal Nazarewicz <mina86@mina86.com>
> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: David Rientjes <rientjes@google.com>

Fair enough.

Reviewed-by: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>

> ---
>  mm/compaction.c | 36 +++++++++++++++++++++++++++++++-----
>  mm/internal.h   | 16 +++++++++++++++-
>  2 files changed, 46 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 41c7005..df0961b 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -270,8 +270,15 @@ static inline bool compact_should_abort(struct compact_control *cc)
>  static bool suitable_migration_target(struct page *page)
>  {
>  	/* If the page is a large free page, then disallow migration */
> -	if (PageBuddy(page) && page_order(page) >= pageblock_order)
> -		return false;
> +	if (PageBuddy(page)) {
> +		/*
> +		 * We are checking page_order without zone->lock taken. But
> +		 * the only small danger is that we skip a potentially suitable
> +		 * pageblock, so it's not worth to check order for valid range.
> +		 */
> +		if (page_order_unsafe(page) >= pageblock_order)
> +			return false;
> +	}
>  
>  	/* If the block is MIGRATE_MOVABLE or MIGRATE_CMA, allow migration */
>  	if (migrate_async_suitable(get_pageblock_migratetype(page)))
> @@ -591,11 +598,23 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
>  			valid_page = page;
>  
>  		/*
> -		 * Skip if free. page_order cannot be used without zone->lock
> -		 * as nothing prevents parallel allocations or buddy merging.
> +		 * Skip if free. We read page order here without zone lock
> +		 * which is generally unsafe, but the race window is small and
> +		 * the worst thing that can happen is that we skip some
> +		 * potential isolation targets.
>  		 */
> -		if (PageBuddy(page))
> +		if (PageBuddy(page)) {
> +			unsigned long freepage_order = page_order_unsafe(page);
> +
> +			/*
> +			 * Without lock, we cannot be sure that what we got is
> +			 * a valid page order. Consider only values in the
> +			 * valid order range to prevent low_pfn overflow.
> +			 */
> +			if (freepage_order > 0 && freepage_order < MAX_ORDER)
> +				low_pfn += (1UL << freepage_order) - 1;
>  			continue;
> +		}
>  
>  		/*
>  		 * Check may be lockless but that's ok as we recheck later.
> @@ -683,6 +702,13 @@ next_pageblock:
>  		low_pfn = ALIGN(low_pfn + 1, pageblock_nr_pages) - 1;
>  	}
>  
> +	/*
> +	 * The PageBuddy() check could have potentially brought us outside
> +	 * the range to be scanned.
> +	 */
> +	if (unlikely(low_pfn > end_pfn))
> +		low_pfn = end_pfn;
> +
>  	acct_isolated(zone, locked, cc);
>  
>  	if (locked)
> diff --git a/mm/internal.h b/mm/internal.h
> index 2c187d2..584cd69 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -171,7 +171,8 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
>   * general, page_zone(page)->lock must be held by the caller to prevent the
>   * page from being allocated in parallel and returning garbage as the order.
>   * If a caller does not hold page_zone(page)->lock, it must guarantee that the
> - * page cannot be allocated or merged in parallel.
> + * page cannot be allocated or merged in parallel. Alternatively, it must
> + * handle invalid values gracefully, and use page_order_unsafe() below.
>   */
>  static inline unsigned long page_order(struct page *page)
>  {
> @@ -179,6 +180,19 @@ static inline unsigned long page_order(struct page *page)
>  	return page_private(page);
>  }
>  
> +/*
> + * Like page_order(), but for callers who cannot afford to hold the zone lock.
> + * PageBuddy() should be checked first by the caller to minimize race window,
> + * and invalid values must be handled gracefully.
> + *
> + * ACCESS_ONCE is used so that if the caller assigns the result into a local
> + * variable and e.g. tests it for valid range before using, the compiler cannot
> + * decide to remove the variable and inline the page_private(page) multiple
> + * times, potentially observing different values in the tests and the actual
> + * use of the result.
> + */
> +#define page_order_unsafe(page)		ACCESS_ONCE(page_private(page))
> +
>  static inline bool is_cow_mapping(vm_flags_t flags)
>  {
>  	return (flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE;
> 


-- 
Thanks.
Zhang Yanfei

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