Re: [PATCH] jffs2 summary allocation

Andrew Morton
2008-04-05T01:47:07Z

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

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