Re: [PATCHSET] block: fix PIO cache coherency bug

[Tejun Heo]

On Fri, Jan 13, 2006 at 07:06:14PM +0000, Russell King wrote:
> On Fri, Jan 13, 2006 at 12:35:24PM -0600, James Bottomley wrote:
> > Perhaps we should take this to linux-arch ... the audience there is well
> > versed in these arcane problems?
> 
> I think you need to wait for Dave Miller to reply and give a definitive
> statement on how his cache coherency model is supposed to work in this
> regard.
> 

Hello, all.

This thread has been dead for quite some time mainly because I didn't
know what to do.  As it is a real outstanding bug bugging people and
Matt Reimer thankfully reminded me[1], I'm giving another shot at
resolving this.

People seem to agree that it is the responsibility of the driver to
make sure read data gets to the page cache page (or whatever kernel
page).  Only driver knows how and when.

The objection raised by James Bottomley is that although syncing the
kernel page is the responsbility of the driver, syncing user page is
not; thus, use of flush_dcache_page() is excessive.  James suggested
use of flush_kernel_dcache_page().

I also asked similar question[2] on lkml and Russell replied that
depending on arch implementation it shouldn't be much of a problem[3].
Another thing to consider is that all other drivers which currently
manage cache coherency use flush_dcache_page().

So, the questions are...

q1. James, besides from the use of flush_dcache_page(), do you agree
    with the block layer kmap/kunmap API?

2. Is flush_kernel_dcache_page() the correct one?

Whether or not flush_kernel_dcache_page() is the one or not, I think
we should first go with flush_dcache_page() as that's what drivers
have been doing upto this point.  Switching from flush_dcache_page()
to flush_kernel_dcache_page() is very easy and should be done in a
separate patch anyway.  No?

Another thing mind is that this problem is not limited block drivers.
All the codes that perform writes to kmap'ed pages take care of
synchronization themselves and the popular choice seems to be
flush_dcache_page().

IMHO, kmap API should have a flag or something to tell it how the page
is being used such that kmap API can take care of synchronization like
dma mapping API does rather than scattering sync code all over the
kernel.  And if that's the right thing to do, some of blk kmap
wrappers can/should be removed.

What do you guys think?

Thanks.

-- 
tejun

[1] http://article.gmane.org/gmane.linux.kernel/379304
[2] http://article.gmane.org/gmane.linux.kernel/360324 (the last question)
[3] http://article.gmane.org/gmane.linux.kernel/365607
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