linux-kernel - Re: [PATCH] jffs2 summary allocation

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Message-Id: <1207357870.3224.89.camel@vader.jdub.homelinux.org>
Date:	Fri, 04 Apr 2008 20:11:10 -0500
From:	Josh Boyer <jwboyer@...il.com>
To:	David Brownell <david-b@...bell.net>
Cc:	linux-arm-kernel@...ts.arm.linux.org.uk,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michael Trimarchi <trimarchimichael@...oo.it>,
	dwmw2@...radead.org, spi-devel-general@...ts.sourceforge.net,
	linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH]  jffs2 summary allocation

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.

I don't see much harm with the actual patch itself, assuming larger
kmallocs work as I think they should.  It does make me wonder if we have
other cases of vmalloc'd buffers being passed to lower drivers using DMA
though.

josh

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