Re: [PATCH v5 2/4] mm/page_alloc: add freepage on isolate pageblock to correct buddy list

[Hui Zhu]

On Mon, Nov 3, 2014 at 4:22 PM, Heesub Shin <heesub.shin@xxxxxxxxxxx> wrote:
> Hello,
>
>
> On 10/31/2014 04:25 PM, Joonsoo Kim wrote:
>>
>> In free_pcppages_bulk(), we use cached migratetype of freepage
>> to determine type of buddy list where freepage will be added.
>> This information is stored when freepage is added to pcp list, so
>> if isolation of pageblock of this freepage begins after storing,
>> this cached information could be stale. In other words, it has
>> original migratetype rather than MIGRATE_ISOLATE.
>>
>> There are two problems caused by this stale information. One is that
>> we can't keep these freepages from being allocated. Although this
>> pageblock is isolated, freepage will be added to normal buddy list
>> so that it could be allocated without any restriction. And the other
>> problem is incorrect freepage accounting. Freepages on isolate pageblock
>> should not be counted for number of freepage.
>>
>> Following is the code snippet in free_pcppages_bulk().
>>
>> /* 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);
>> }
>>
>> As you can see above snippet, current code already handle second problem,
>> incorrect freepage accounting, by re-fetching pageblock migratetype
>> through is_migrate_isolate_page(page). But, because this re-fetched
>> information isn't used for __free_one_page(), first problem would not be
>> solved. This patch try to solve this situation to re-fetch pageblock
>> migratetype before __free_one_page() and to use it for __free_one_page().
>>
>> In addition to move up position of this re-fetch, this patch use
>> optimization technique, re-fetching migratetype only if there is
>> isolate pageblock. Pageblock isolation is rare event, so we can
>> avoid re-fetching in common case with this optimization.
>>
>> This patch also correct migratetype of the tracepoint output.
>>
>> Cc: <stable@xxxxxxxxxxxxxxx>
>> Acked-by: Minchan Kim <minchan@xxxxxxxxxx>
>> Acked-by: Michal Nazarewicz <mina86@xxxxxxxxxx>
>> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
>> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
>> ---
>>   mm/page_alloc.c |   13 ++++++++-----
>>   1 file changed, 8 insertions(+), 5 deletions(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index f7a867e..6df23fe 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -725,14 +725,17 @@ static void free_pcppages_bulk(struct zone *zone,
>> int count,
>>                         /* must delete as __free_one_page list manipulates
>> */
>>                         list_del(&page->lru);
>>                         mt = get_freepage_migratetype(page);
>> +                       if (unlikely(has_isolate_pageblock(zone))) {
>
>
> How about adding an additional check for 'mt == MIGRATE_MOVABLE' here? Then,
> most of get_pageblock_migratetype() calls could be avoided while the
> isolation is in progress. I am not sure this is the case on memory
> offlining. How do you think?

I think the reason is that this "mt" may be not the right value of this page.
It is set without zone->lock.

Thanks,
Hui

>
>> +                               mt = get_pageblock_migratetype(page);
>> +                               if (is_migrate_isolate(mt))
>> +                                       goto skip_counting;
>> +                       }
>> +                       __mod_zone_freepage_state(zone, 1, mt);
>> +
>> +skip_counting:
>>                         /* 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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