Re: [PATCH] jffs2 summary allocation

Josh Boyer
2008-04-05T01:11:14Z

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