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

Hugh Dickins
2008-08-14T12:47:17Z

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