linux-kernel - Re: pud_bad vs pud

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Date:	Thu, 5 Feb 2009 21:09:04 +0000 (GMT)
From:	Hugh Dickins <hugh@...itas.com>
To:	wli@...ementarian.org
cc:	Ingo Molnar <mingo@...e.hu>, Jeremy Fitzhardinge <jeremy@...p.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: pud_bad vs pud_bad

On Thu, 5 Feb 2009, wli@...ementarian.org wrote:
> On Thu, 5 Feb 2009, wli@...ementarian.org wrote:
> >> The RW bit needs to be allowed to become read-only for hugetlb COW.
> >> Changing it over to the 32-bit method is a bugfix by that token.
> 
> On Thu, Feb 05, 2009 at 08:14:42PM +0000, Hugh Dickins wrote:
> > If there's a bugfix to be made there, of course I'm in favour:
> > but how come we've never seen such a bug?  hugetlb COW has been
> > around for a year or two by now, hasn't it?
> 
> We can tell from the code that a write-protected pte mapping of a
> 1GB hugetlb page would be flagged as bad. It must not be called on
> ptes mapping hugetlb pages if they're not getting flagged.

Ah, I see what you mean now.  Yes, the hugetlb case goes its own way
and doesn't normally hit those p??_bad() macro/inlines; but we got
caught out in follow_page() a year ago, a bad looked huge or a
huge looked bad, but I forget the details at this instant.

Hugh
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