Re: [PATCH 03/10] mm/page_alloc: handle page on pcp correctly if it's pageblock is isolated

[Vlastimil Babka]

On 07/07/2014 10:25 AM, Gioh Kim wrote:
> 
> 
> 2014-07-04 오후 4:57, Joonsoo Kim 쓴 글:
>> If pageblock of page on pcp are isolated now, we should free it to isolate
>> buddy list to prevent future allocation on it. But current code doesn't
>> do this.
> 
> I think it is strage that pcp can have isolated page.
> I remember that The purpose of pcp is having frequently used pages (hot for cache),
> but isolated page is not for frequently allocated and freed.

It can happen that a page was placed on pcplist *before* the pageblock
was isolated.

> (Above is my guess. It can be wrong. I know CMA and hot-plug features are using isolated pages
> so that I guess isolated pages are not for frequently allocated and freed memory.)
> 
> Anyway if isolated page is not good for pcp, what about preventing isolated page from inserting pcp?
> I think fix of free_hot_cold_page() can be one of the preventing ways.
> The free_hot_cold_page() checks migratetype and inserts CMA page into pcp.
> Am I correct?
> 
> If I am correct I think inserting CMA page into pcp is harmful for both of CMA and pcp.
> If CMA page is on pcp it will be used frequently and prevent making contiguous memory.
> What about avoiding CMA page for pcp like following?

CMA is not the only user of memory isolation. There's also memory
hot-remove. Also CMA tries to make the CMA-marked pages usable as normal
MOVABLE pages, until a CMA allocation is needed. Here you would restrict
their usage outside of pcplists.

> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 8c9eeec..1cbb816 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1413,13 +1413,8 @@ void free_hot_cold_page(struct page *page, bool cold)
>           * areas back if necessary. Otherwise, we may have to free
>           * excessively into the page allocator
>           */
> -       if (migratetype >= MIGRATE_PCPTYPES) {
> -               if (unlikely(is_migrate_isolate(migratetype))) {
> -                       free_one_page(zone, page, pfn, 0, migratetype);
> -                       goto out;
> -               }
> -               migratetype = MIGRATE_MOVABLE;
> -       }
> +       if (migratetype > MIGRATE_PCPTYPES)
> +               goto out;

Certainly not, for the reasons above and also:
- by changing >= to > you stopped handling RESERVE pages specially
- by removing free_one_page() call you are now leaking the CMA and
ISOLATE pages.

>          pcp = &this_cpu_ptr(zone->pageset)->pcp;
>          if (!cold)
> 
> 
>>
>> Moreover, there is a freepage counting problem on current code. Although
>> pageblock of page on pcp are isolated now, it could go normal buddy list,
>> because get_onpcp_migratetype() will return non-isolate migratetype.
> 
> I cannot find get_onpcp_migratetype() in v3.16.0-rc3 and also cannot google it.
> Is it typo?
> 
>> In this case, we should do either adding freepage count or changing
>> migratetype to MIGRATE_ISOLATE, but, current code do neither.
>>
>> This patch fixes these two problems by handling pageblock migratetype
>> before calling __free_one_page(). And, if we find the page on isolated
>> pageblock, change migratetype to MIGRATE_ISOLATE to prevent future
>> allocation of this page and freepage counting problem.
>>
>> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
>> ---
>>    mm/page_alloc.c |   14 ++++++++------
>>    1 file changed, 8 insertions(+), 6 deletions(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index aeb51d1..99c05f7 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -719,15 +719,17 @@ static void free_pcppages_bulk(struct zone *zone, int count,
>>    			page = list_entry(list->prev, struct page, lru);
>>    			/* must delete as __free_one_page list manipulates */
>>    			list_del(&page->lru);
>> -			mt = get_freepage_migratetype(page);
>> +
>> +			if (unlikely(is_migrate_isolate_page(page))) {
>> +				mt = MIGRATE_ISOLATE;
>> +			} else {
>> +				mt = get_freepage_migratetype(page);
>> +				__mod_zone_freepage_state(zone, 1, mt);
>> +			}
>> +
>>    			/* MIGRATE_MOVABLE list may include MIGRATE_RESERVEs */
>>    			__free_one_page(page, page_to_pfn(page), zone, 0, mt);
>>    			trace_mm_page_pcpu_drain(page, 0, mt);
>> -			if (likely(!is_migrate_isolate_page(page))) {
>> -				__mod_zone_page_state(zone, NR_FREE_PAGES, 1);
>> -				if (is_migrate_cma(mt))
>> -					__mod_zone_page_state(zone, NR_FREE_CMA_PAGES, 1);
>> -			}
>>    		} while (--to_free && --batch_free && !list_empty(list));
>>    	}
>>    	spin_unlock(&zone->lock);
>>
> 
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