linux-kernel - Re: [PATCH -v8 3/4] Enable the MS_ASYNC functionality in sys

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Message-Id: <200801241236.01114.nickpiggin@yahoo.com.au>
Date:	Thu, 24 Jan 2008 12:36:00 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Anton Salikhmetov <salikhmetov@...il.com>, linux-mm@...ck.org,
	jakob@...hought.net, linux-kernel@...r.kernel.org,
	valdis.kletnieks@...edu, riel@...hat.com, ksm@...dk,
	staubach@...hat.com, jesper.juhl@...il.com, a.p.zijlstra@...llo.nl,
	akpm@...ux-foundation.org, protasnb@...il.com, miklos@...redi.hu,
	r.e.wolff@...wizard.nl, hidave.darkstar@...il.com,
	hch@...radead.org
Subject: Re: [PATCH -v8 3/4] Enable the MS_ASYNC functionality in sys_msync()

On Thursday 24 January 2008 04:05, Linus Torvalds wrote:
> On Wed, 23 Jan 2008, Anton Salikhmetov wrote:
> > +
> > +		if (pte_dirty(*pte) && pte_write(*pte)) {
>
> Not correct.
>
> You still need to check "pte_present()" before you can test any other
> bits. For a non-present pte, none of the other bits are defined, and for
> all we know there might be architectures out there that require them to
> be non-dirty.
>
> As it is, you just possibly randomly corrupted the pte.
>
> Yeah, on all architectures I know of, it the pte is clear, neither of
> those tests will trigger, so it just happens to work, but it's still
> wrong.

Probably it can fail for !present nonlinear mappings on many
architectures.

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