Re: [RFC PATCH 1/3] mm: when stealing freepages, also take pages created by splitting buddy page

[Joonsoo Kim]

On Thu, Dec 04, 2014 at 06:12:56PM +0100, Vlastimil Babka wrote:
> When __rmqueue_fallback() is called to allocate a page of order X, it will
> find a page of order Y >= X of a fallback migratetype, which is different from
> the desired migratetype. With the help of try_to_steal_freepages(), it may
> change the migratetype (to the desired one) also of:
> 
> 1) all currently free pages in the pageblock containing the fallback page
> 2) the fallback pageblock itself
> 3) buddy pages created by splitting the fallback page (when Y > X)
> 
> These decisions take the order Y into account, as well as the desired
> migratetype, with the goal of preventing multiple fallback allocations that
> could e.g. distribute UNMOVABLE allocations among multiple pageblocks.
> 
> Originally, decision for 1) has implied the decision for 3). Commit
> 47118af076f6 ("mm: mmzone: MIGRATE_CMA migration type added") changed that
> (probably unintentionally) so that the buddy pages in case 3) are always
> changed to the desired migratetype, except for CMA pageblocks.
> 
> Commit fef903efcf0c ("mm/page_allo.c: restructure free-page stealing code and
> fix a bug") did some refactoring and added a comment that the case of 3) is
> intended. Commit 0cbef29a7821 ("mm: __rmqueue_fallback() should respect
> pageblock type") removed the comment and tried to restore the original behavior
> where 1) implies 3), but due to the previous refactoring, the result is instead
> that only 2) implies 3) - and the conditions for 2) are less frequently met
> than conditions for 1). This may increase fragmentation in situations where the
> code decides to steal all free pages from the pageblock (case 1)), but then
> gives back the buddy pages produced by splitting.
> 
> This patch restores the original intended logic where 1) implies 3). During
> testing with stress-highalloc from mmtests, this has shown to decrease the
> number of events where UNMOVABLE and RECLAIMABLE allocations steal from MOVABLE
> pageblocks, which can lead to permanent fragmentation. It has increased the
> number of events when MOVABLE allocations steal from UNMOVABLE or RECLAIMABLE
> pageblocks, but these are fixable by sync compaction and thus less harmful.
> 
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---
>  mm/page_alloc.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 616a2c9..548b072 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1105,12 +1105,10 @@ static int try_to_steal_freepages(struct zone *zone, struct page *page,
>  
>  		/* Claim the whole block if over half of it is free */
>  		if (pages >= (1 << (pageblock_order-1)) ||
> -				page_group_by_mobility_disabled) {
> -
> +				page_group_by_mobility_disabled)
>  			set_pageblock_migratetype(page, start_type);
> -			return start_type;
> -		}
>  
> +		return start_type;
>  	}
>  
>  	return fallback_type;

change_ownership on tracepoint will be wrong with this change.

Thanks.
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