Re: [PATCH 4/6] mm/hwpoison: don't set migration type twice to avoid hold heavy contend zone->lock

[Wanpeng Li <liwanp@linux.vnet.ibm.com>]

Hi Naoya,
On Thu, Aug 22, 2013 at 03:06:05PM -0400, Naoya Horiguchi wrote:
>On Thu, Aug 22, 2013 at 05:48:25PM +0800, Wanpeng Li wrote:
>> Set pageblock migration type will hold zone->lock which is heavy contended 
>> in system to avoid race. However, soft offline page will set pageblock 
>> migration type twice during get page if the page is in used, not hugetlbfs 
>> page and not on lru list. There is unnecessary to set the pageblock migration
>> type and hold heavy contended zone->lock again if the first round get page 
>> have already set the pageblock to right migration type.
>
>Can we use get_pageblock_migratetype() outside zone->lock?
>

I think the trick here is migration type is MIGRATE_ISOLATE. There 
are two parts can change MIGRATE_ISOLATE except hwpoison. One is 
memory hoplug, however, we hold lock_memory_hotplug() which avoid 
race. The second is CMA which umovable page allocation requst can't 
fallback to. So I think it's safe here.

Regards,
Wanpeng Li 

>There are surely some users which call this function outside
>zone->lock like free_hot_cold_pages(), __free_pages, etc.,
>but I think that there's a race window where migratetype is
>updated just after get_pageblock_migratetype() check.
>
>> Signed-off-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>
>> ---
>>  mm/memory-failure.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>> 
>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>> index 6bfd51e..3bfb45f 100644
>> --- a/mm/memory-failure.c
>> +++ b/mm/memory-failure.c
>> @@ -1413,7 +1413,8 @@ static int __get_any_page(struct page *p, unsigned long pfn, int flags)
>>  	 * was free. This flag should be kept set until the source page
>>  	 * is freed and PG_hwpoison on it is set.
>>  	 */
>> -	set_migratetype_isolate(p, true);
>> +	if (get_pageblock_migratetype(p) == MIGRATE_ISOLATE)
>
>You meant '!=', right?
>
>Thanks,
>Naoya Horiguchi
>
>> +		set_migratetype_isolate(p, true);
>>  	/*
>>  	 * When the target page is a free hugepage, just remove it
>>  	 * from free hugepage list.
>> -- 
>> 1.8.1.2
>>
>
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