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Date:	Thu, 03 Apr 2014 17:36:53 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
CC:	Mel Gorman <mgorman@...e.de>, Hugh Dickins <hughd@...gle.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Yong-Taek Lee <ytk.lee@...sung.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH v3] mm/page_alloc: fix freeing of MIGRATE_RESERVE migratetype
 pages

On 03/25/2014 02:47 PM, Bartlomiej Zolnierkiewicz wrote:
>
> Hi,
>
> On Friday, March 21, 2014 03:16:31 PM Vlastimil Babka wrote:
>> On 03/06/2014 06:35 PM, Bartlomiej Zolnierkiewicz wrote:
>>> Pages allocated from MIGRATE_RESERVE migratetype pageblocks
>>> are not freed back to MIGRATE_RESERVE migratetype free
>>> lists in free_pcppages_bulk()->__free_one_page() if we got
>>> to free_pcppages_bulk() through drain_[zone_]pages().
>>> The freeing through free_hot_cold_page() is okay because
>>> freepage migratetype is set to pageblock migratetype before
>>> calling free_pcppages_bulk().
>>
>> I think this is somewhat misleading and got me confused for a while.
>> It's not about the call path of free_pcppages_bulk(), but about the
>> fact that rmqueue_bulk() has been called at some point to fill up the
>> pcp lists, and had to resort to __rmqueue_fallback(). So, going through
>> free_hot_cold_page() might give you correct migratetype for the last
>> page freed, but the pcp lists may still contain misplaced pages from
>> earlier rmqueue_bulk().
>
> Ok, you're right.  I'll fix this.
>
>>> If pages of MIGRATE_RESERVE
>>> migratetype end up on the free lists of other migratetype
>>> whole Reserved pageblock may be later changed to the other
>>> migratetype in __rmqueue_fallback() and it will be never
>>> changed back to be a Reserved pageblock.  Fix the issue by
>>> moving freepage migratetype setting from rmqueue_bulk() to
>>> __rmqueue[_fallback]() and preserving freepage migratetype
>>> as an original pageblock migratetype for MIGRATE_RESERVE
>>> migratetype pages.
>>
>> Actually wouldn't the easiest solution to this particular problem to
>> check current pageblock migratetype in try_to_steal_freepages() and
>> disallow changing it. However I agree that preventing the misplaced page
>> in the first place would be even better.
>>
>>> The problem was introduced in v2.6.31 by commit ed0ae21
>>> ("page allocator: do not call get_pageblock_migratetype()
>>> more than necessary").
>>>
>>> Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
>>> Reported-by: Yong-Taek Lee <ytk.lee@...sung.com>
>>> Cc: Marek Szyprowski <m.szyprowski@...sung.com>
>>> Cc: Mel Gorman <mgorman@...e.de>
>>> Cc: Hugh Dickins <hughd@...gle.com>
>>> ---
>>> v2:
>>> - updated patch description, there is no __zone_pcp_update()
>>>     in newer kernels
>>> v3:
>>> - set freepage migratetype in __rmqueue[_fallback]()
>>>     instead of rmqueue_bulk() (per Mel's request)
>>>
>>>    mm/page_alloc.c |   27 ++++++++++++++++++---------
>>>    1 file changed, 18 insertions(+), 9 deletions(-)
>>>
>>> Index: b/mm/page_alloc.c
>>> ===================================================================
>>> --- a/mm/page_alloc.c	2014-03-06 18:10:21.884422983 +0100
>>> +++ b/mm/page_alloc.c	2014-03-06 18:10:27.016422895 +0100
>>> @@ -1094,7 +1094,7 @@ __rmqueue_fallback(struct zone *zone, in
>>>    	struct free_area *area;
>>>    	int current_order;
>>>    	struct page *page;
>>> -	int migratetype, new_type, i;
>>> +	int migratetype, new_type, mt = start_migratetype, i;
>>
>> A better naming would help, "mt" and "migratetype" are the same thing
>> and it gets too confusing.
>
> Well, yes, though 'mt' is short and the check code is consistent with
> the corresponding code in rmqueue_bulk().
>
> Do you have a proposal for a better name for this variable?
>
>>>
>>>    	/* Find the largest possible block of pages in the other list */
>>>    	for (current_order = MAX_ORDER-1; current_order >= order;
>>> @@ -1125,6 +1125,14 @@ __rmqueue_fallback(struct zone *zone, in
>>>    			expand(zone, page, order, current_order, area,
>>>    			       new_type);
>>>
>>> +			if (IS_ENABLED(CONFIG_CMA)) {
>>> +				mt = get_pageblock_migratetype(page);
>>> +				if (!is_migrate_cma(mt) &&
>>> +				    !is_migrate_isolate(mt))
>>> +					mt = start_migratetype;
>>> +			}
>>> +			set_freepage_migratetype(page, mt);
>>> +
>>>    			trace_mm_page_alloc_extfrag(page, order, current_order,
>>>    				start_migratetype, migratetype, new_type);
>>>
>>> @@ -1147,7 +1155,9 @@ static struct page *__rmqueue(struct zon
>>>    retry_reserve:
>>>    	page = __rmqueue_smallest(zone, order, migratetype);
>>>
>>> -	if (unlikely(!page) && migratetype != MIGRATE_RESERVE) {
>>> +	if (likely(page)) {
>>> +		set_freepage_migratetype(page, migratetype);
>>
>> Are you sure that here the checking of of CMA and ISOLATE is not needed?
>
> CMA and ISOLATE migratetype pages are always put back on the correct
> free lists (since set_freepage_migratetype() sets freepage migratetype
> to the original one for CMA and ISOLATE migratetype pages) and
> __rmqueue_smallest() can take page only from the 'migratetype' free
> list.

Actually, this is true also for the __rmqueue_fallback() case. So we can 
do without get_pageblock_migratetype() completely. In fact, Joonsoo 
already posted such patch in "[PATCH 3/7] mm/page_alloc: move 
set_freepage_migratetype() to better place", see:
http://lkml.org/lkml/2014/1/9/33

I've updated and improved this and will send shortly along with some 
DEBUG_VM checks to test easier that this is indeed the case. Testing 
from the CMA people is welcome.

Vlastimil

> + It was suggested to do it this way by Mel.
>
>> Did the original rmqueue_bulk() have this checking only for the
>> __rmqueue_fallback() case? Why wouldn't the check already be only in
>> __rmqueue_fallback() then?
>
> Probably because of historical reasons.  The rmqueue_bulk() contained
> set_page_private() call when CMA was introduced and added the special
> handling for CMA and ISOLATE migratetype pages, please see commit
> 47118af ("mm: mmzone: MIGRATE_CMA migration type added").
>
>>> +	} else if (migratetype != MIGRATE_RESERVE) {
>>>    		page = __rmqueue_fallback(zone, order, migratetype);
>>>
>>>    		/*
>>> @@ -1174,7 +1184,7 @@ static int rmqueue_bulk(struct zone *zon
>>>    			unsigned long count, struct list_head *list,
>>>    			int migratetype, int cold)
>>>    {
>>> -	int mt = migratetype, i;
>>> +	int i;
>>>
>>>    	spin_lock(&zone->lock);
>>>    	for (i = 0; i < count; ++i) {
>>> @@ -1195,16 +1205,15 @@ static int rmqueue_bulk(struct zone *zon
>>>    			list_add(&page->lru, list);
>>>    		else
>>>    			list_add_tail(&page->lru, list);
>>> +		list = &page->lru;
>>>    		if (IS_ENABLED(CONFIG_CMA)) {
>>> -			mt = get_pageblock_migratetype(page);
>>> +			int mt = get_pageblock_migratetype(page);
>>>    			if (!is_migrate_cma(mt) && !is_migrate_isolate(mt))
>>>    				mt = migratetype;
>>> +			if (is_migrate_cma(mt))
>>> +				__mod_zone_page_state(zone, NR_FREE_CMA_PAGES,
>>> +						      -(1 << order));
>>>    		}
>>> -		set_freepage_migratetype(page, mt);
>>> -		list = &page->lru;
>>> -		if (is_migrate_cma(mt))
>>> -			__mod_zone_page_state(zone, NR_FREE_CMA_PAGES,
>>> -					      -(1 << order));
>>>    	}
>>>    	__mod_zone_page_state(zone, NR_FREE_PAGES, -(i << order));
>>>    	spin_unlock(&zone->lock);
>
> Best regards,
> --
> Bartlomiej Zolnierkiewicz
> Samsung R&D Institute Poland
> Samsung Electronics
>
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