linux-kernel - Re: [RFC][PATCH 2/6] mm: Change flush_tlb_range() to take an mm

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Date:	Wed, 02 Mar 2011 15:58:59 -0500
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>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-mm@...ck.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Miller <davem@...emloft.net>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Mel Gorman <mel@....ul.ie>, Nick Piggin <npiggin@...nel.dk>,
	Russell King <rmk@....linux.org.uk>,
	Chris Metcalf <cmetcalf@...era.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [RFC][PATCH 2/6] mm: Change flush_tlb_range() to take an mm_struct

On 03/02/2011 02:19 PM, Linus Torvalds wrote:

>> There are various reasons that we need to flush TLBs _after_ freeing
>> the page-tables themselves. For some architectures (x86 among others)
>> this serializes against (both hardware and software) page table
>> walkers like gup_fast().
>
> This part of the changelog also makes no sense what-so-ever. It's
> actively wrong.
>
> On x86, we absolutely *must* do the TLB flush _before_ we release the
> page tables. So your commentary is actively wrong and misleading.
>
> The order has to be:
>   - clear the page table entry, queue the page to be free'd
>   - flush the TLB
>   - free the page (and page tables)
>
> and nothing else is correct, afaik. So the changelog is pure and utter
> garbage. I didn't look at what the patch actually changed.

The patch seems to preserve the correct behaviour.

The changelog should probably read something along the
lines of:

"There are various reasons that we need to flush TLBs _after_
  clearing the page-table entries themselves."
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