linux-kernel - Re: [PATCH 4/7] nandsim: Don't use PF

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Date:	Wed, 25 Nov 2009 09:13:22 +0200
From:	Adrian Hunter <adrian.hunter@...ia.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
CC:	"Bityutskiy Artem (Nokia-D/Helsinki)" <Artem.Bityutskiy@...ia.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Woodhouse <David.Woodhouse@...el.com>,
	"linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>
Subject: Re: [PATCH 4/7] nandsim: Don't use PF_MEMALLOC

KOSAKI Motohiro wrote:
>> KOSAKI Motohiro wrote:
>>> Hi
>>>
>>> Thank you for this useful comments.
>>>
>>>>> I vaguely remember Adrian (CCed) did this on purpose. This is for the
>>>>> case when nandsim emulates NAND flash on top of a file. So there are 2
>>>>> file-systems involved: one sits on top of nandsim (e.g. UBIFS) and the
>>>>> other owns the file which nandsim uses (e.g., ext3).
>>>>>
>>>>> And I really cannot remember off the top of my head why he needed
>>>>> PF_MEMALLOC, but I think Adrian wanted to prevent the direct reclaim
>>>>> path to re-enter, say UBIFS, and cause deadlock. But I'd thing that all
>>>>> the allocations in vfs_read()/vfs_write() should be GFP_NOFS, so that
>>>>> should not be a probelm?
>>>>>
>>>> Yes it needs PF_MEMALLOC to prevent deadlock because there can be a
>>>> file system on top of nandsim which, in this case, is on top of another
>>>> file system.
>>>>
>>>> I do not see how mempools will help here.
>>>>
>>>> Please offer an alternative solution.
>>> I have few questions.
>>>
>>> Can you please explain more detail? Another stackable filesystam
>>> (e.g. ecryptfs) don't have such problem. Why nandsim have its issue?
>>> What lock cause deadlock?
>> The file systems are not stacked.  One is over nandsim, which nandsim
>> does not know about because it is just a lowly NAND device, and, with
>> the file cache option, one file system below to provide the file cache.
>>
>> The deadlock is the kernel writing out dirty pages to the top file system
>> which writes to nandsim which writes to the bottom file system which
>> allocates memory which causes dirty pages to be written out to the top
>> file system, which tries to write to nandsim => deadlock.
> 
> You mean you want to prevent pageout() instead reclaim itself?

Yes

> Dropping filecache seems don't make recursive call, right?

Yes

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