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

[Ralf Baechle <ralf@linux-mips.org>]

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