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Subject: Re: [PATCH] mm, page_alloc: fix core hung in free_pcppages_bulk()
To:     David Rientjes <rientjes@google.com>
Cc:     akpm@linux-foundation.org, mhocko@suse.com, vbabka@suse.cz,
        david@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        vinmenon@codeaurora.org
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From:   Charan Teja Kalla <charante@codeaurora.org>
Message-ID: <aa382119-0679-0adc-d9ea-833fa7ed7525@codeaurora.org>
Date:   Tue, 11 Aug 2020 18:31:05 +0530
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Thanks David.

On 8/11/2020 1:06 AM, David Rientjes wrote:
> On Mon, 10 Aug 2020, Charan Teja Reddy wrote:
> 
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index e4896e6..25e7e12 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -3106,6 +3106,7 @@ static void free_unref_page_commit(struct page *page, unsigned long pfn)
>>  	struct zone *zone = page_zone(page);
>>  	struct per_cpu_pages *pcp;
>>  	int migratetype;
>> +	int high;
>>  
>>  	migratetype = get_pcppage_migratetype(page);
>>  	__count_vm_event(PGFREE);
>> @@ -3128,8 +3129,19 @@ static void free_unref_page_commit(struct page *page, unsigned long pfn)
>>  	pcp = &this_cpu_ptr(zone->pageset)->pcp;
>>  	list_add(&page->lru, &pcp->lists[migratetype]);
>>  	pcp->count++;
>> -	if (pcp->count >= pcp->high) {
>> -		unsigned long batch = READ_ONCE(pcp->batch);
>> +	high = READ_ONCE(pcp->high);
>> +	if (pcp->count >= high) {
>> +		int batch;
>> +
>> +		batch = READ_ONCE(pcp->batch);
>> +		/*
>> +		 * For non-default pcp struct values, high is always
>> +		 * greater than the batch. If high < batch then pass
>> +		 * proper count to free the pcp's list pages.
>> +		 */
>> +		if (unlikely(high < batch))
>> +			batch = min(pcp->count, batch);
>> +
>>  		free_pcppages_bulk(zone, batch, pcp);
>>  	}
>>  }
> 
> I'm wondering if a fix to free_pcppages_bulk() is more appropriate here 
> because the count passed into it seems otherwise fragile if this results 
> in a hung core?
> 

Agree that the free_pcppages_bulk() is appropriate place to fix and it
actually much cleaner. Raised V2:
https://patchwork.kernel.org/patch/11709225/

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