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

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Message-Id: <20091124194532.AFC2.A69D9226@jp.fujitsu.com>
Date:	Tue, 24 Nov 2009 19:46:25 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Adrian Hunter <adrian.hunter@...ia.com>
Cc:	kosaki.motohiro@...fujitsu.com,
	"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

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?



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