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Date:	Fri, 4 Apr 2008 18:46:15 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Kyungmin Park" <kmpark@...radead.org>
Cc:	"Josh Boyer" <jwboyer@...il.com>,
	"David Brownell" <david-b@...bell.net>,
	linux-kernel@...r.kernel.org, linux-mtd@...ts.infradead.org,
	"Michael Trimarchi" <trimarchimichael@...oo.it>,
	spi-devel-general@...ts.sourceforge.net, dwmw2@...radead.org,
	linux-arm-kernel@...ts.arm.linux.org.uk
Subject: Re: [PATCH] jffs2 summary allocation

On Sat, 5 Apr 2008 10:29:25 +0900 "Kyungmin Park" <kmpark@...radead.org> wrote:

> On Sat, Apr 5, 2008 at 10:11 AM, Josh Boyer <jwboyer@...il.com> 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.
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