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Date: Fri, 16 Feb 2001 16:54:44 +0100
From: Manfred Spraul <manfred@colorfullife.com>
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To: Jamie Lokier <lk@tantalophile.demon.co.uk>
CC: Linus Torvalds <torvalds@transmeta.com>, linux-kernel@vger.kernel.org
Subject: Re: x86 ptep_get_and_clear question
In-Reply-To: <3A8C499A.E0370F63@colorfullife.com> <Pine.LNX.4.10.10102151702320.12656-100000@penguin.transmeta.com> <20010216151839.A3989@pcep-jamie.cern.ch> <3A8D4045.F8F27782@colorfullife.com> <20010216162741.A4284@pcep-jamie.cern.ch>
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Jamie Lokier wrote:
> 
> /* mprotect.c */
>         entry = ptep_get_and_clear(pte);
>         set_pte(pte, pte_modify(entry, newprot));
> 
> I.e. the only code with the race condition is code which explicitly
> clears the dirty bit, in vmscan.c.
> 
> Do you see any possibility of losing a dirty bit here?
>
Of course.
Just check the output after preprocessing.
It's 
	int entry;
	entry = *pte;
	entry &= ~_PAGE_CHG_MASK;
	entry |= pgprot_val(newprot)
	*pte = entry;

We need
	atomic_clear_mask (_PAGE_CHG_MASK, pte);
	atomic_set_mask (pgprot_val(newprot), *pte);

for multi threaded apps.

> If not, there's no need for the intricate "gather" or "double scan"
> schemes for mprotect() and it can stay as fast as possible.
>
Correct, but we need a platform specific "update_pte", and perhaps
update_begin, update_end hooks (empty on i386) for other archs.

--
	Manfred
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