linux-kernel - Re: page allocator bug in 3.16?

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Message-ID: <54255EA5.3030207@redhat.com>
Date:	Fri, 26 Sep 2014 08:40:05 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Rob Clark <robdclark@...il.com>,
	Thomas Hellstrom <thellstrom@...are.com>
CC:	Chuck Ebbert <cebbert.lkml@...il.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Hugh Dickens <hughd@...gle.com>,
	Linux kernel <linux-kernel@...r.kernel.org>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	linux-mm <linux-mm@...ck.org>, Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Shaohua Li <shli@...nel.org>, Ingo Molnar <mingo@...nel.org>
Subject: Re: page allocator bug in 3.16?

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On 09/26/2014 08:28 AM, Rob Clark wrote:
> On Fri, Sep 26, 2014 at 6:45 AM, Thomas Hellstrom
> <thellstrom@...are.com> wrote:
>> On 09/26/2014 12:40 PM, Chuck Ebbert wrote:
>>> On Fri, 26 Sep 2014 09:15:57 +0200 Thomas Hellstrom
>>> <thellstrom@...are.com> wrote:
>>> 
>>>> On 09/26/2014 01:52 AM, Peter Hurley wrote:
>>>>> On 09/25/2014 03:35 PM, Chuck Ebbert wrote:
>>>>>> There are six ttm patches queued for 3.16.4:
>>>>>> 
>>>>>> drm-ttm-choose-a-pool-to-shrink-correctly-in-ttm_dma_pool_shrink_scan.patch
>>>>>>
>>>>>> 
drm-ttm-fix-handling-of-ttm_pl_flag_topdown-v2.patch
>>>>>> drm-ttm-fix-possible-division-by-0-in-ttm_dma_pool_shrink_scan.patch
>>>>>>
>>>>>> 
drm-ttm-fix-possible-stack-overflow-by-recursive-shrinker-calls.patch
>>>>>> drm-ttm-pass-gfp-flags-in-order-to-avoid-deadlock.patch 
>>>>>> drm-ttm-use-mutex_trylock-to-avoid-deadlock-inside-shrinker-functions.patch
>>>>>
>>>>>> 
Thanks for info, Chuck.
>>>>> 
>>>>> Unfortunately, none of these fix TTM dma allocation doing
>>>>> CMA dma allocation, which is the root problem.
>>>>> 
>>>>> Regards, Peter Hurley
>>>> The problem is not really in TTM but in CMA, There was a guy
>>>> offering to fix this in the CMA code but I guess he didn't
>>>> probably because he didn't receive any feedback.
>>>> 
>>> Yeah, the "solution" to this problem seems to be "don't enable
>>> CMA on x86". Maybe it should even be disabled in the config
>>> system.
>> Or, as previously suggested, don't use CMA for order 0 (single
>> page) allocations....
> 
> On devices that actually need CMA pools to arrange for memory to be
> in certain ranges, I think you probably do want to have order 0
> pages come from the CMA pool.
> 
> Seems like disabling CMA on x86 (where it should be unneeded) is
> the better way, IMO

CMA has its uses on x86. For example, CMA is used to allocate 1GB huge
pages.

There may also be people with devices that do not scatter-gather, and
need a large physically contiguous buffer, though there should be
relatively few of those on x86.

I suspect it makes most sense to do DMA allocations up to PAGE_ORDER
through the normal allocator on x86, and only invoking CMA for larger
allocations.

- -- 
All rights reversed
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