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

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Message-Id: <20061020.152247.111203913.davem@davemloft.net>
Date:	Fri, 20 Oct 2006 15:22:47 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	torvalds@...l.org
Cc:	ralf@...ux-mips.org, nickpiggin@...oo.com.au, akpm@...l.org,
	linux-kernel@...r.kernel.org, anemo@....ocn.ne.jp,
	linux-arch@...r.kernel.org, schwidefsky@...ibm.com,
	James.Bottomley@...elEye.com
Subject: Re: [PATCH 1/3] Fix COW D-cache aliasing on fork

From: Linus Torvalds <torvalds@...l.org>
Date: Fri, 20 Oct 2006 15:02:39 -0700 (PDT)

> On Fri, 20 Oct 2006, Ralf Baechle wrote:
> > 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.
> 
> I just suspect it means a _buggy_ fork.
> 
> It so happens (I think), that fork is big enough that it probably flushes 
> the L1 cache _anyway_. 

My understanding is that this works because in Ralf's original patch
(which is the context in which he is removing the flush_cache_mm()
call), he uses kmap()/kunmap() to map the page(s) being accessed at a
kernel virtual address which will fall into the same cache color as
the user virtual address --> no alias problems.

Since he does this for every page touched on the kernel side during
dup_mmap(), the existing flush_cache_mm() call in dup_mmap() does in
fact become redundant.
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