Re: [PATCH] mm: page_idle_get_page() does not need zone_lru_lock

[Vlastimil Babka <vbabka@suse.cz>]

On 12/07/2016 12:08 PM, Vladimir Davydov wrote:
> Hello,
> 
> On Mon, Dec 05, 2016 at 09:55:10PM -0800, Hugh Dickins wrote:
>> Rechecking PageLRU() after get_page_unless_zero() may have value, but
>> holding zone_lru_lock around that serves no useful purpose: delete it.
> 
> IIRC this lock/unlock was added on purpose, by request from Minchan. It
> serves as a barrier that guarantees that all page fields (specifically
> ->mapping in case of anonymous pages) have been properly initialized by
> the time we pass it to rmap_walk(). Here's a reference to the thread
> where this problem was discussed:
> 
>   http://lkml.kernel.org/r/<20150430082531.GD21771@blaptop>

Um OK, but then there should have been a comment explaining why the lock
is there :/

Vlastimil

>>
>> Signed-off-by: Hugh Dickins <hughd@google.com>
>> ---
>>
>>  mm/page_idle.c |    4 ----
>>  1 file changed, 4 deletions(-)
>>
>> --- 4.9-rc8/mm/page_idle.c	2016-10-02 16:24:33.000000000 -0700
>> +++ linux/mm/page_idle.c	2016-12-05 19:44:32.646625435 -0800
>> @@ -30,7 +30,6 @@
>>  static struct page *page_idle_get_page(unsigned long pfn)
>>  {
>>  	struct page *page;
>> -	struct zone *zone;
>>  
>>  	if (!pfn_valid(pfn))
>>  		return NULL;
>> @@ -40,13 +39,10 @@ static struct page *page_idle_get_page(u
>>  	    !get_page_unless_zero(page))
>>  		return NULL;
>>  
>> -	zone = page_zone(page);
>> -	spin_lock_irq(zone_lru_lock(zone));
>>  	if (unlikely(!PageLRU(page))) {
>>  		put_page(page);
>>  		page = NULL;
>>  	}
>> -	spin_unlock_irq(zone_lru_lock(zone));
>>  	return page;
>>  }
>>  
> 
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