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

[Vlastimil Babka <vbabka@suse.cz>]

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.

> 
>>  
>>  	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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