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

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Message-ID: <561B980C.6060809@suse.cz>
Date:	Mon, 12 Oct 2015 13:22:52 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	"Kirill A. Shutemov" <kirill@...temov.name>,
	yalin wang <yalin.wang2010@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	mgorman@...hsingularity.net, mhocko@...e.com,
	David Rientjes <rientjes@...gle.com>, js1304@...il.com,
	"Kirill A. Shutemov" <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 12:05 PM, Kirill A. Shutemov wrote:
> On Mon, Oct 12, 2015 at 03:58:51PM +0800, yalin wang wrote:
>>
>>> On Oct 12, 2015, at 15:38, Vlastimil Babka <vbabka@...e.cz> wrote:
>>>
>>> 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…
>> agree with your comments,
>> do i need send a new patch for this ?

I'd guess no need to, Andrew can edit the patch?

>
> Looks like a two patches to me: memory leak and removing always-true part
> of condifition.

Yeah but I'd expect both would be in the end folded into the buggy patch 
in -mm?

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