linux-kernel - Re: [RFC] mm: fix a BUG, the page is allocated 2 times

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Message-ID: <561B6379.2070407@suse.cz>
Date:	Mon, 12 Oct 2015 09:38:33 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	yalin wang <yalin.wang2010@...il.com>, akpm@...ux-foundation.org,
	mgorman@...hsingularity.net, mhocko@...e.com, rientjes@...gle.com,
	js1304@...il.com, kirill.shutemov@...ux.intel.com,
	hannes@...xchg.org, alexander.h.duyck@...hat.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC] mm: fix a BUG, the page is allocated 2 times

On 10/12/2015 04:40 AM, yalin wang wrote:
> Remove unlikely(order), because we are sure order is not zero if
> code reach here, also add if (page == NULL), only allocate page again if
> __rmqueue_smallest() failed or alloc_flags & ALLOC_HARDER == 0

The second mentioned change is actually more important as it removes a 
memory leak! Thanks for catching this. The problem is in patch 
mm-page_alloc-reserve-pageblocks-for-high-order-atomic-allocations-on-demand.patch 
and seems to have been due to a change in the last submitted version to 
make sure the tracepoint is called.

> Signed-off-by: yalin wang <yalin.wang2010@...il.com>
> ---
>   mm/page_alloc.c | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 0d6f540..de82e2c 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2241,13 +2241,13 @@ struct page *buffered_rmqueue(struct zone *preferred_zone,
>   		spin_lock_irqsave(&zone->lock, flags);
>
>   		page = NULL;
> -		if (unlikely(order) && (alloc_flags & ALLOC_HARDER)) {
> +		if (alloc_flags & ALLOC_HARDER) {
>   			page = __rmqueue_smallest(zone, order, MIGRATE_HIGHATOMIC);
>   			if (page)
>   				trace_mm_page_alloc_zone_locked(page, order, migratetype);
>   		}
> -
> -		page = __rmqueue(zone, order, migratetype, gfp_flags);
> +		if (page == NULL)

"if (!page)" is more common and already used below.
We could skip the check for !page in case we don't go through the 
ALLOC_HARDER branch, but I guess it's not worth the goto, and hopefully 
the compiler is smart enough anyway...

> +			page = __rmqueue(zone, order, migratetype, gfp_flags);
>   		spin_unlock(&zone->lock);
>   		if (!page)
>   			goto failed;
>

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