From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp1.linux-foundation.org ([140.211.169.13]) by bombadil.infradead.org with esmtps (Exim 4.68 #1 (Red Hat Linux)) id 1JhxUo-0006PK-Kp for linux-mtd@lists.infradead.org; Sat, 05 Apr 2008 01:47:07 +0000 Date: Fri, 4 Apr 2008 18:46:15 -0700 From: Andrew Morton To: "Kyungmin Park" Subject: Re: [PATCH] jffs2 summary allocation Message-Id: <20080404184615.deaf3122.akpm@linux-foundation.org> In-Reply-To: <9c9fda240804041829r5a768b39n340926485aa12687@mail.gmail.com> References: <713171.37644.qm@web26213.mail.ukl.yahoo.com> <200804041609.41092.david-b@pacbell.net> <1207351282.3224.79.camel@vader.jdub.homelinux.org> <200804041658.38499.david-b@pacbell.net> <1207357870.3224.89.camel@vader.jdub.homelinux.org> <9c9fda240804041829r5a768b39n340926485aa12687@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: linux-kernel@vger.kernel.org, David Brownell , linux-mtd@lists.infradead.org, Michael Trimarchi , spi-devel-general@lists.sourceforge.net, Josh Boyer , dwmw2@infradead.org, linux-arm-kernel@lists.arm.linux.org.uk List-Id: Linux MTD discussion mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Sat, 5 Apr 2008 10:29:25 +0900 "Kyungmin Park" wrote: > On Sat, Apr 5, 2008 at 10:11 AM, Josh Boyer wrote: > > On Fri, 2008-04-04 at 16:58 -0700, David Brownell wrote: > > > On Friday 04 April 2008, Josh Boyer wrote: > > > > > > > > > ... This means specifically that you may _not_ use the > > > > > memory/addresses returned from vmalloc() for DMA. ... > > > > > > > > > > So I'm rather surprised to see *ANY* kernel code trying to do > > > > > that. That rule has been in effect for many, many years now. > > > > > > > > I don't think it was intentional. You're going through several layers > > > > here: > > > > > > > > JFFS2 -> mtd parts -> mtd dataflash -> atmel_spi. > > > > > > > > Typically MTD drivers aren't doing DMAs to flash and JFFS2 has no idea > > > > which particular chip driver is being used because it's abstracted by > > > > MTD. > > > > > > That's true ... although I can imagine using DMA to > > > avoid dcache trashing if its setup cost is low enough, > > > with either NAND or NOR chips. > > > > > > Still: in this context vmalloc() is wrong. > > > > Agreed. One issue is that the summary code allocates a buffer that > > equals the eraseblock size of the underlying MTD device. For larger > > NAND chips, that may be up to 256KiB. I believe this is within the > > allowable kmalloc size for most architectures these days, but the > > summary code is 3 years old and was likely expecting a smaller limit. > > And there is always the question on whether finding that much contiguous > > memory will be an issue. Yes. This is why I'm reluctant to whizz this patch into 2.6.25. It'll break more than it fixes. > In MLC chips it goes up to 512KiB. It means it can't allocate the > eraseblock size memory with kmalloc(). > In ARM environment I can't see the 256KiB or more memory allocation > with kmalloc(). > So I now changed the kmalloc eraseblock to vmalloc at both jffs2 and mtd-utils. Does this eraseblock really really really need to be a single virtually-contiguous hunk of kernel memory? Or was that just easy to do at the time? This problem comes up pretty often. Rather than open-coding it yet again it'd be nice to have a little bit of library code which manages an array of pages and which has accessors for common operations like read/write-u8/u16/u32/u64, memset, memcpy, etc. Then again, given that this memory is often fed into IO subsystems, perhaps we should do this by adding more accessors and helpers to scatterlists/sg_table. Unfortunately they're not presently well set up for random access.