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SubjectRe: [PATCH v2 3/6] perf/core: Fix arch_perf_get_page_size()
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()?

[...]
> + pmdp = pmd_offset_lockless(pudp, pud, addr);
> + pmd = READ_ONCE(*pmdp);
> + if (!pmd_present(pmd))
> return 0;
>
> + if (pmd_leaf(pmd))
> + return pmd_leaf_size(pmd);
>

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