Re: [PATCH] mm/hotplug: fix a drain pcp bug when offline pages

[Xishi Qiu <qiuxishi@huawei.com>]

On 2013/8/2 2:59, Cody P Schafer wrote:

> On 08/01/2013 02:18 AM, Xishi Qiu wrote:
>> __offline_pages()
>>     start_isolate_page_range()
>>        set_migratetype_isolate()
>>           set_pageblock_migratetype() -> this pageblock will be marked as MIGRATE_ISOLATE
>>           move_freepages_block() -> pages in PageBuddy will be moved into MIGRATE_ISOLATE list
>>           drain_all_pages() -> drain PCP
>>              free_pcppages_bulk()
>>                 mt = get_freepage_migratetype(page); -> PCP's migratetype is not MIGRATE_ISOLATE
>>                 __free_one_page(page, zone, 0, mt); -> so PCP will not be freed into into MIGRATE_ISOLATE list
>>
>> In this case, the PCP may be allocated again, because they are not in
>> PageBuddy's MIGRATE_ISOLATE list. This will cause offline_pages failed.
>>
>> Signed-off-by: Xishi Qiu <qiuxishi@huawei.com>
>> ---
>>   mm/page_alloc.c     |   10 ++++++----
>>   mm/page_isolation.c |   15 ++++++++++++++-
>>   2 files changed, 20 insertions(+), 5 deletions(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index b100255..d873471 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -965,11 +965,13 @@ int move_freepages(struct zone *zone,
>>           }
>>
>>           order = page_order(page);
>> -        list_move(&page->lru,
>> -              &zone->free_area[order].free_list[migratetype]);
>> -        set_freepage_migratetype(page, migratetype);
>> +        if (get_freepage_migratetype(page) != migratetype) {
>> +            list_move(&page->lru,
>> +                &zone->free_area[order].free_list[migratetype]);
>> +            set_freepage_migratetype(page, migratetype);
>> +            pages_moved += 1 << order;
>> +        }
>>           page += 1 << order;
>> -        pages_moved += 1 << order;
> 
> So this looks like it changes the return from move_freepages() to be the "pages moved" from "the pages now belonging to the passed migrate type".
> 
> The user of move_freepages_block()'s return value (and thus the return value of move_freepages()) in mm/page_alloc.c expects that it is the original meaning. The users in page_isolation.c expect it is the new meaning. Those need to be reconciled.
> 

>>       }
>>
>>       return pages_moved;
>> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
>> index 383bdbb..ba1afc9 100644
>> --- a/mm/page_isolation.c
>> +++ b/mm/page_isolation.c
>> @@ -65,8 +65,21 @@ out:
>>       }
>>
>>       spin_unlock_irqrestore(&zone->lock, flags);
>> -    if (!ret)
>> +
>> +    if (!ret) {
>>           drain_all_pages();
>> +        /*
>> +         * When drain_all_pages() frees cached pages into the buddy
>> +         * system, it uses the stale migratetype cached in the
>> +         * page->index field, so try to move free pages to ISOLATE
>> +         * list again.
>> +         */
>> +        spin_lock_irqsave(&zone->lock, flags);
>> +        nr_pages = move_freepages_block(zone, page, MIGRATE_ISOLATE);
>> +        __mod_zone_freepage_state(zone, -nr_pages, migratetype);
>> +        spin_unlock_irqrestore(&zone->lock, flags);
>> +    }
>> +
> 
> Could we teach drain_all_pages() to use the right migrate type instead (or add something similar that does)? (pages could be reallocated between the drain_all_pages() and move_freepages_block()).
> 

Sorry, I made a mistake. __test_page_isolated_in_pageblock() will catch the pages and move 
them into MIGRATE_ISOLATE list. So the code is right.

Thanks,
Xishi Qiu

>>       return ret;
>>   }
>>
> 
> .
> 


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