linux-kernel - Re: [PATCH] mm: page_alloc: High-order per-cpu page allocator v4

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Date:   Thu, 1 Dec 2016 15:27:28 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Linux-MM <linux-mm@...ck.org>,
        Linux-Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: page_alloc: High-order per-cpu page allocator v4

On 12/01/2016 03:24 PM, Mel Gorman wrote:
> On Thu, Dec 01, 2016 at 02:41:29PM +0100, Vlastimil Babka wrote:
>> On 12/01/2016 01:24 AM, Mel Gorman wrote:
>>
>> ...
>>
>>
>> Hmm I think that if this hits, we don't decrease count/increase nr_freed and
>> pcp->count will become wrong.
>
> Ok, I think you're right but I also think it's relatively trivial to fix
> with
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 94808f565f74..8777aefc1b8e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1134,13 +1134,13 @@ static void free_pcppages_bulk(struct zone *zone, int count,
>  			if (unlikely(isolated_pageblocks))
>  				mt = get_pageblock_migratetype(page);
>
> +			nr_freed += (1 << order);
> +			count -= (1 << order);
>  			if (bulkfree_pcp_prepare(page))
>  				continue;
>
>  			__free_one_page(page, page_to_pfn(page), zone, order, mt);
>  			trace_mm_page_pcpu_drain(page, order, mt);
> -			nr_freed += (1 << order);
> -			count -= (1 << order);
>  		} while (count > 0 && --batch_free && !list_empty(list));
>  	}
>  	spin_unlock(&zone->lock);
>
>> And if we are unlucky/doing full drain, all
>> lists will get empty, but as count stays e.g. 1, we loop forever on the
>> outer while()?
>>
>
> Potentially yes. Granted the system is already in a bad state as pages
> are being freed in a bad or unknown state but we haven't halted the
> system for that in the past.
>
>> BTW, I think there's a similar problem (but not introduced by this patch) in
>> rmqueue_bulk() and its
>>
>>     if (unlikely(check_pcp_refill(page)))
>>             continue;
>>
>
> Potentially yes. It's outside the scope of this patch but it needs
> fixing.
>
> If you agree with the above fix, I'll roll it into a v5 and append
> another patch for this issue.

Yeah, looks fine. Thanks.