linux-kernel - Re: [PATCH V2] mm, page_alloc: fix core hung in free_pcppages

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Date:   Thu, 13 Aug 2020 21:51:29 +0530
From:   Charan Teja Kalla <charante@...eaurora.org>
To:     Michal Hocko <mhocko@...e.com>
Cc:     akpm@...ux-foundation.org, vbabka@...e.cz, david@...hat.com,
        rientjes@...gle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, vinmenon@...eaurora.org
Subject: Re: [PATCH V2] mm, page_alloc: fix core hung in free_pcppages_bulk()

Thanks Michal for comments.

On 8/13/2020 5:11 PM, Michal Hocko wrote:
> On Tue 11-08-20 18:28:23, Charan Teja Reddy wrote:
> [...]
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index e4896e6..839039f 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -1304,6 +1304,11 @@ static void free_pcppages_bulk(struct zone *zone, int count,
>>  	struct page *page, *tmp;
>>  	LIST_HEAD(head);
>>  
>> +	/*
>> +	 * Ensure proper count is passed which otherwise would stuck in the
>> +	 * below while (list_empty(list)) loop.
>> +	 */
>> +	count = min(pcp->count, count);
>>  	while (count) {
>>  		struct list_head *list;
> 
> 
> How does this prevent the race actually?

This doesn't prevent the race. This only fixes the core hung(as this is
called with spin_lock_irq()) caused by the race condition. This core
hung is because of incorrect count value is passed to the
free_pcppages_bulk() function.

The actual race should be fixed by David's suggestion (isolate, online
and undo isolation).

Something needs to be improved in commit message? May be like below:
s/The following race is observed with the repeated ... / Cpu core hung
is observed as a result of race with the use case of repeated...

 Don't we need something like
> the following instead?
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e028b87ce294..45bcc7ba37c4 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1317,9 +1317,16 @@ static void free_pcppages_bulk(struct zone *zone, int count,
>  		 * lists
>  		 */
>  		do {
> +			bool looped = false;

IIUC, this looped will always be initialzed to false thus never jumped
to free.
But I think I got your idea that looping of the pcp lists for any pages.
If not found despite MIGRATE_PCPTYPES count lists are traversed, just
break the loop. Does this checking really required? Shouldn't pcp->count
tells the same whether any pages left in the pcp lists?

> +
>  			batch_free++;
> -			if (++migratetype == MIGRATE_PCPTYPES)
> +			if (++migratetype == MIGRATE_PCPTYPES) {
> +				if (looped)
> +					goto free;
> +
>  				migratetype = 0;
> +				looped = true;
> +			}
>  			list = &pcp->lists[migratetype];
>  		} while (list_empty(list));
>  
> @@ -1352,6 +1359,7 @@ static void free_pcppages_bulk(struct zone *zone, int count,
>  		} while (--count && --batch_free && !list_empty(list));
>  	}
>  
> +free:
>  	spin_lock(&zone->lock);
>  	isolated_pageblocks = has_isolate_pageblock(zone);
>  
> 

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