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Date:   Thu, 26 Nov 2020 14:06:19 +0100
From:   Peter Zijlstra <>
To:     Matthew Wilcox <>
Subject: Re: [PATCH v2 3/6] perf/core: Fix arch_perf_get_page_size()

On Thu, Nov 26, 2020 at 12:56:06PM +0000, Matthew Wilcox wrote:
> On Thu, Nov 26, 2020 at 01:42:07PM +0100, Peter Zijlstra wrote:
> > +	pgdp = pgd_offset(mm, addr);
> > +	pgd = READ_ONCE(*pgdp);
> I forget how x86-32-PAE maps to Linux's PGD/P4D/PUD/PMD scheme, but
> according to volume 3, section 4.4.2, PAE paging uses a 64-bit PDE, so
> whether a PDE is a PGD or a PMD, we're only reading it with READ_ONCE
> rather than the lockless-retry method used by ptep_get_lockless().
> So it's potentially racy?  Do we need a pmdp_get_lockless() or
> pgdp_get_lockless()?

Oh gawd... this isn't new here though, right? Current gup_fast also gets
that wrong, if it is in deed wrong.

I suppose it's a race far more likely today, with THP and all, than it
ever was back then.

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