linux-kernel - Re: [PATCH v2 3/6] perf/core: Fix arch_perf_get_page

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Date:   Thu, 26 Nov 2020 14:06:19 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     kan.liang@...ux.intel.com, mingo@...nel.org, acme@...nel.org,
        mark.rutland@....com, alexander.shishkin@...ux.intel.com,
        jolsa@...hat.com, eranian@...gle.com, christophe.leroy@...roup.eu,
        npiggin@...il.com, linuxppc-dev@...ts.ozlabs.org,
        mpe@...erman.id.au, will@...nel.org, aneesh.kumar@...ux.ibm.com,
        sparclinux@...r.kernel.org, davem@...emloft.net,
        catalin.marinas@....com, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, ak@...ux.intel.com,
        dave.hansen@...el.com, kirill.shutemov@...ux.intel.com
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.