linux-kernel - Re: [PATCH 1/3] Fix COW D-cache aliasing on fork

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Date:	Fri, 20 Oct 2006 22:49:16 +0100
From:	Ralf Baechle <ralf@...ux-mips.org>
To:	Linus Torvalds <torvalds@...l.org>
Cc:	David Miller <davem@...emloft.net>, nickpiggin@...oo.com.au,
	Andrew Morton <akpm@...l.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	anemo@....ocn.ne.jp, linux-arch@...r.kernel.org,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	James Bottomley <James.Bottomley@...elEye.com>
Subject: Re: [PATCH 1/3] Fix COW D-cache aliasing on fork

On Fri, Oct 20, 2006 at 01:10:59PM -0700, Linus Torvalds wrote:

> Ok, this sounds sane.
> 
> What should we do about this? How does this patch look to people?
> 
> (Totally untested, and I'm not sure we should even do that whole 
> "oldmm->mm_users" test, but I'm throwing it out here for discussion, in 
> case it matters for performance. The second D$ flush should obviously be 
> unnecessary for the common unthreaded case, which is why none of this has 
> mattered historically, I think).
> 
> Comments? We need ARM, MIPS, sparc and S390 at the very least to sign off 
> on this, and somebody to write a nice explanation for the changelog (and 
> preferably do this through -mm too).

As a minimal solution your patch would work for MIPS but performance would be
suboptimal.

With my D-cache alias series applied the flush_cache_mm() in dup_mmap()
becomes entirely redundant.  When I delete the call (not part of my patchset)
it means 12% faster fork.  But I'm not proposing this for 2.6.19.

Note this does not make the flush_cache_mm() on process termination
redundant ...

  Ralf
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