linux-kernel - Re: [PATCH] x86: Optimize tail handling for copy

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Date:	Fri, 1 Aug 2008 15:30:26 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Vitaly Mayatskikh <v.mayatskih@...il.com>
cc:	linux-kernel@...r.kernel.org, Andi Kleen <andi@...stfloor.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] x86: Optimize tail handling for copy_user

On Sat, 2 Aug 2008, Vitaly Mayatskikh wrote:
> 
> Well, I have sanity test for it (at http://people.redhat.com/vmayatsk/copy_user_x8664/), 
> but unfortunately it shows what I want to see, not the truth. I'm going
> to make a large regression testing next week.

I was actually thinking more along the lines of actually doing a 
test-suite with the few most relevant system calls (to catch the few cases 
that matter), and judicious use of mmap/munmap to make sure to trigger it 
in-kernel.

It probably doesn't really need all that many system calls to trigger all 
the relevant paths. A "read()" should trigger the "to_user()" case, and a 
write() to a file should trigger the 'from_user_nocache()" case. And while 
the "from_user()" case with zeroing migth be harder to see (because all 
_normal_ users should also look at the error case and return EFAULT), I 
think there are a few cases where we just depend on the zeroing.

Doing a

	git grep '	copy_from_user('

(that's a tab in that grep thing) shows at least the termios code doing 
it, for example. But fewer cases than I expected.

			Linus
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