linux-kernel - Re: [PATCH 05/31] x86/mm: Reduce tlb flushes from ptep_set_access

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Date:	Thu, 25 Oct 2012 23:57:01 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 05/31] x86/mm: Reduce tlb flushes from ptep_set_access_flags()

On 10/25/2012 10:56 PM, Linus Torvalds wrote:

> Guess what? If you want to optimize the function to not do remote TLB
> flushes, then just do that! None of the garbage. Just change the
>
>      flush_tlb_page(vma, address);
>
> line to
>
>      __flush_tlb_one(address);

That may not even be needed.  Apparently Intel chips
automatically flush an entry from the TLB when it
causes a page fault.  I assume AMD chips do the same,
because flush_tlb_fix_spurious_fault evaluates to
nothing on x86.

> and it should damn well work. Because everything I see about
> "flush_remote" looks just wrong, wrong, wrong.

Are there architectures where we do need to flush
remote TLBs on upgrading the permissions on a PTE?

Because that is what the implementation in
pgtable-generic.c seems to be doing as well...

> And if there really is some reason for that whole flush_remote
> braindamage, then we have much bigger problems, namely the fact that
> we've broken the documented semantics of that function, and we're
> doing various other things that are completely and utterly invalid
> unless the above semantics hold.

Want to just remove the TLB flush entirely and see
if anything breaks in 3.8-rc1?

 From reading the code again, it looks like things
should indeed work ok.

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