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

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Date:	Mon, 16 Apr 2012 10:32:41 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	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>
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>,
	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>
Subject: Re: [PATCH 4/4] ARM: remove consistent dma region and use common
 vmalloc range for dma allocations

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?


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