From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.nokia.com ([192.100.122.230] helo=mgw-mx03.nokia.com) by bombadil.infradead.org with esmtps (Exim 4.69 #1 (Red Hat Linux)) id 1NCu0r-0006dj-3w for linux-mtd@lists.infradead.org; Tue, 24 Nov 2009 11:56:58 +0000 Message-ID: <4B0BC9E3.6070504@nokia.com> Date: Tue, 24 Nov 2009 13:56:19 +0200 From: Adrian Hunter MIME-Version: 1.0 To: ext KOSAKI Motohiro Subject: Re: [PATCH 4/7] nandsim: Don't use PF_MEMALLOC References: <1258988417.18407.44.camel@localhost> <4B0AEA33.3010306@nokia.com> <20091124194532.AFC2.A69D9226@jp.fujitsu.com> In-Reply-To: <20091124194532.AFC2.A69D9226@jp.fujitsu.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Cc: "Bityutskiy Artem \(Nokia-D/Helsinki\)" , David Woodhouse , LKML , linux-mm , "linux-mtd@lists.infradead.org" , Andrew Morton List-Id: Linux MTD discussion mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , ext 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.