Re: [PATCH] mm, page_alloc: fix the duplicate save/ressave irq

[zhong jiang <zhongjiang@huawei.com>]

On 2017/3/13 18:08, Vlastimil Babka wrote:
> On 03/13/2017 09:31 AM, Vlastimil Babka wrote:
>> On 03/13/2017 09:02 AM, zhongjiang wrote:
>>> From: zhong jiang <zhongjiang@huawei.com>
>>>
>>> when commit 374ad05ab64d ("mm, page_alloc: only use per-cpu allocator for irq-safe requests")
>>> introduced to the mainline, free_pcppages_bulk irq_save/resave to protect
>>> the IRQ context. but drain_pages_zone fails to clear away the irq. because
>>> preempt_disable have take effect. so it safely remove the code.
>>>
>>> Fixes: 374ad05ab64d ("mm, page_alloc: only use per-cpu allocator for irq-safe requests")
>>> Signed-off-by: zhong jiang <zhongjiang@huawei.com>
>>> ---
>>>  mm/page_alloc.c | 3 ---
>>>  1 file changed, 3 deletions(-)
>>>
>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>> index 05c3956..7b16095 100644
>>> --- a/mm/page_alloc.c
>>> +++ b/mm/page_alloc.c
>>> @@ -2294,11 +2294,9 @@ void drain_zone_pages(struct zone *zone, struct per_cpu_pages *pcp)
>>>   */
>>>  static void drain_pages_zone(unsigned int cpu, struct zone *zone)
>>>  {
>>> -	unsigned long flags;
>>>  	struct per_cpu_pageset *pset;
>>>  	struct per_cpu_pages *pcp;
>>>  
>>> -	local_irq_save(flags);
>>>  	pset = per_cpu_ptr(zone->pageset, cpu);
>> NAK. we have to make sure that pset corresponds to the cpu we are
>> running on.
> Sorry, I was thinking about other callers, such as drain_local_pages(),
> but seems like all have the cpu pinned prerequisity.
>
> But do we know that there can't be an interrupt updating pcp->count
> between the moment we read it and we call free_pcppages_bulk? This
> should be also discussed in the changelog. Also the "Fixes:" tag is not
> necessary for a performance optimization.
 The title is misunderstanding. it is just for performence optimization.
 I will resent it in v2.

 Thanks
 zhongjiang
>>>  
>>>  	pcp = &pset->pcp;
>>> @@ -2306,7 +2304,6 @@ static void drain_pages_zone(unsigned int cpu, struct zone *zone)
>>>  		free_pcppages_bulk(zone, pcp->count, pcp);
>>>  		pcp->count = 0;
>>>  	}
>>> -	local_irq_restore(flags);
>>>  }
>>>  
>>>  /*
>>>
>
> .
>


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