linux-kernel - Re: [PATCH 4/4] ARM: remove consistent dma region and use common vmalloc range for dma allocations

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Date:	Tue, 17 Apr 2012 14:18:04 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Minchan Kim <minchan@...nel.org>
CC:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>,
	Hugh Dickins <hughd@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	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>,
	Chunsang Jeong <chunsang.jeong@...aro.org>,
	Krishna Reddy <vdumpa@...dia.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Hiroshi Doyu <hdoyu@...dia.com>,
	Subash Patel <subashrp@...il.com>, kosaki.motohiro@...il.com
Subject: Re: [PATCH 4/4] ARM: remove consistent dma region and use common
 vmalloc range for dma allocations

(4/15/12 9:32 PM), Minchan Kim wrote:
> On 04/14/2012 03:38 AM, Russell King - ARM Linux wrote:
>> On Fri, Apr 13, 2012 at 04:05:50PM +0200, Marek Szyprowski wrote:
>>> This patch changes dma-mapping subsystem to use generic vmalloc areas
>>> for all consistent dma allocations. This increases the total size limit
>>> of the consistent allocations and removes platform hacks and a lot of
>>> duplicated code.
>>
>> NAK. I don't think you appreciate the contexts from which the dma coherent
>> code can be called from, and the reason why we pre-allocate the page
>> tables (so that IRQ-based allocations work.)
>>
>> The vmalloc region doesn't allow that because page tables are allocated
>> using GFP_KERNEL not GFP_ATOMIC.
>>
>> Sorry.
>>
>
> Off-topic.
>
> I don't know why vmalloc functions have gfp_t argument.
> As Russel pointed out, we allocates page tables with GFP_KERNEL regardless of gfp_t passed.
> It means gfp_t passed is useless.
> I see there are many cases calling __vmalloc with GFP_NOFS, even GFP_ATOMIC. Then, it could end up deadlocking in reclaim context or schedule bug.
> I'm not sure why we can't see such bugs until now.
> If I didn't miss something, Shouldn't we fix it?

I believe it should be fixed. of course. :)


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