linux-kernel - Re: [PATCH 3/3] mm,generic: only flush the local TLB in ptep_set_access

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Message-ID: <CA+55aFyS_iJcKz=-zSDK+bjYiNeEzy4T5FrrGL8HBsxTOSwpJQ@mail.gmail.com>
Date:	Fri, 26 Oct 2012 11:48:48 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	Ingo Molnar <mingo@...nel.org>, Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.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
Subject: Re: [PATCH 3/3] mm,generic: only flush the local TLB in ptep_set_access_flags

On Fri, Oct 26, 2012 at 11:46 AM, Rik van Riel <riel@...hat.com> wrote:
>
> The function ptep_set_access_flags is only ever used to upgrade
> access permissions to a page.

NOTE: It's *not* "access permissions". It's "access flags".

Big difference. This is not about permissions at all.

The access flags are the Accessed and Dirty bits. And the dirty bit is
never *cleared* by this function, it's only ever potentially set.
That, together with the fact that the accessed flag is "best effort"
rather than exact, is what makes this function so special to begin
with.

                    Linus
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