linux-kernel - Re: [PATCH v2] mm: dmapool: use provided gfp flags for all dma_alloc

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Date:	Tue, 20 Nov 2012 15:27:27 -0500
From:	Jason Cooper <jason@...edaemon.net>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Marek Szyprowski <m.szyprowski@...sung.com>,
	linux-arm-kernel@...ts.infradead.org,
	linaro-mm-sig@...ts.linaro.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Arnd Bergmann <arnd@...db.de>, Soren Moch <smoch@....de>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>,
	Andrew Lunn <andrew@...n.ch>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Michal Hocko <mhocko@...e.cz>, Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH v2] mm: dmapool: use provided gfp flags for all
 dma_alloc_coherent() calls

On Tue, Nov 20, 2012 at 11:33:25AM -0800, Andrew Morton wrote:
> On Tue, 20 Nov 2012 15:31:45 +0100
> Marek Szyprowski <m.szyprowski@...sung.com> wrote:
> 
> > dmapool always calls dma_alloc_coherent() with GFP_ATOMIC flag,
> > regardless the flags provided by the caller. This causes excessive
> > pruning of emergency memory pools without any good reason. Additionaly,
> > on ARM architecture any driver which is using dmapools will sooner or
> > later  trigger the following error: 
> > "ERROR: 256 KiB atomic DMA coherent pool is too small!
> > Please increase it with coherent_pool= kernel parameter!".
> > Increasing the coherent pool size usually doesn't help much and only
> > delays such error, because all GFP_ATOMIC DMA allocations are always
> > served from the special, very limited memory pool.
> > 
> 
> Is this problem serious enough to justify merging the patch into 3.7? 
> And into -stable kernels?

kirkwood and orion5x currently have the following code in their early
init:

/*
 * Some Kirkwood devices allocate their coherent buffers from atomic
 * context. Increase size of atomic coherent pool to make sure such the
 * allocations won't fail.
 */
init_dma_coherent_pool_size(SZ_1M);

We have a pending patch to do the same for mvebu (new armv7 Marvell
SoCs).  There is at least one reported real world case where even the
above isn't sufficient [1].

thx,

Jason.

[1] http://www.spinics.net/lists/arm-kernel/msg205495.html
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