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

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Date:	Fri, 26 Oct 2012 05:34:02 -0700
From:	Michel Lespinasse <walken@...gle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Rik van Riel <riel@...hat.com>,
	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 Thu, Oct 25, 2012 at 9:23 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Oct 25, 2012 at 8:57 PM, Rik van Riel <riel@...hat.com> wrote:
>>
>> 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.
>
> Yes. It's not architected as far as I know, though. But I agree, it's
> possible - even likely - we could avoid TLB flushing entirely on x86.

Actually, it is architected on x86. This was first described in the
intel appnote 317080 "TLBs, Paging-Structure Caches, and Their
Invalidation", last paragraph of section 5.1. Nowadays, the same
contents are buried somewhere in Volume 3 of the architecture manual
(in my copy: 4.10.4.1 Operations that Invalidate TLBs and
Paging-Structure Caches)

> If you want to try it, I would seriously suggest you do it as a
> separate commit though, just in case.
>
>> Are there architectures where we do need to flush
>> remote TLBs on upgrading the permissions on a PTE?
>
> I *suspect* that whole TLB flush just magically became an SMP one
> without anybody ever really thinking about it.

I would be very worried about assuming every non-x86 arch has similar
TLB semantics. However, if their fault handlers always invalidate TLB
for pages that get spurious faults, then skipping the remote
invalidation would be fine. (I believe this is what
tlb_fix_spurious_fault() is for ?)

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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