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

[Matthew Wilcox]

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);
>