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

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Message-ID: <20201126125606.GR4327@casper.infradead.org>
Date:   Thu, 26 Nov 2020 12:56:06 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Peter Zijlstra <peterz@...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 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);
>