linux-kernel - Re: O_DIRECT patch for processors with VIPT cache for mainline kernel (specifically arm in our case)

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Message-ID: <20081120171713.GB27398@linux-mips.org>
Date:	Thu, 20 Nov 2008 17:17:14 +0000
From:	Ralf Baechle <ralf@...ux-mips.org>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	linux-fsdevel@...r.kernel.org, Naval Saini <navalnovel@...il.com>,
	linux-arch@...r.kernel.org,
	linux-arm-kernel@...ts.arm.linux.org.uk,
	linux-kernel@...r.kernel.org, naval.saini@....com,
	James Bottomley <James.Bottomley@...senPartnership.com>
Subject: Re: O_DIRECT patch for processors with VIPT cache for mainline
	kernel (specifically arm in our case)

On Thu, Nov 20, 2008 at 08:27:19AM -0700, Matthew Wilcox wrote:

> I'm not quite sure why you need kmap_coherent().  If a page is mapped into
> userspace, you can find what address it's mapped to from
> page->mapping->i_mmap and page->index.  OTOH, that's potentially

Even if we know the userspace address of a page we do not necessarily have
a usable mapping for kernel purposes.  The userspace mapping might be r/o
when we need r/w or it might be in another process.  kmap_coherent takes
the job of creating a r/w mapping on a suitable kernel virtual address
that will avoid any aliases.

> page->mapping->i_mmap and page->index.  OTOH, that's potentially
> expensive since you need to grab the spinlock, and unless you have all
> user addresses coherent with each other (like parisc does), you need to
> figure out which process to be coherent with.

Having all userspace addresses of a page across all processes coherent with
each other is the only practicable solution in Linux; at least I don't think
how otherwise and within the currently kernel framework a platform could
sanely handle userspace-userspace aliases.  So we're talking about extending
this to cover userspace-kernelspace aliases.

The original reason for the introduction of kmap_coherent was avoiding
a cache alias in when a multi-threaded process forks.  The issue has been
debated on lkml in 2006 as part of my submission of a patchset under the
subject of "Fix COW D-cache aliasing on fork".  The description is somewhat
lengthy so I omit it here.

One of the ugly parts of kmap_coherent() is that it cannot be used safely
if the page has been marked as dirty by flush_dcache_page(); the callers
know about this and deal with it.

> I know James Bottomley did an experiment (and did an OLS presentation
> ...) on unmapping the entire page cache and greatly expanding the kmap
> area to do just this kind of thing.  I think he even got a speedup.

The speedup is no surprise.

  Ralf
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