linux-kernel - Re: [rfc][patch] mm: dirty page accounting race fix

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Date:	Thu, 14 Aug 2008 13:47:17 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
cc:	Nick Piggin <npiggin@...e.de>,
	Linux Memory Management List <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [rfc][patch] mm: dirty page accounting race fix

On Thu, 14 Aug 2008, Peter Zijlstra wrote:
> On Thu, 2008-08-14 at 12:55 +0100, Hugh Dickins wrote:
> 
> > But I got a bit distracted: mprotect's change_pte_range is
> > traditionally where the pte_modify operation has been split up into
> > stages on some arches, that really can be restricting permissions
> > and needs to tread carefully.  Now I go to look there, I see its
> > 		/*
> > 		 * Avoid taking write faults for pages we know to be
> > 		 * dirty.
> > 		 */
> > 		if (dirty_accountable && pte_dirty(ptent))
> > 			ptent = pte_mkwrite(ptent);
> > 
> > and get rather worried: isn't that likely to be giving write permission
> > to a pte in a vma we are precisely taking write permission away from?
> 
> Exactly, we do that because the page is already dirty, therefore we do
> not need to trap on write to mark it dirty - at least, that was the idea
> behind this optimization.

I realized that was the intended optimization, what I'd missed is that
dirty_accountable can only be true there if (vma->vm_flags & VM_WRITE):
that's checked in vma_wants_writenotify(), which is how dirty_accountable
gets to be set.

So those lines are okay, panic over, phew.

Hugh
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