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From: Matthew Wilcox <willy@infradead.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: kan.liang@linux.intel.com, mingo@kernel.org, acme@kernel.org,
	mark.rutland@arm.com, alexander.shishkin@linux.intel.com,
	jolsa@redhat.com, eranian@google.com,
	christophe.leroy@csgroup.eu, npiggin@gmail.com,
	linuxppc-dev@lists.ozlabs.org, mpe@ellerman.id.au,
	will@kernel.org, aneesh.kumar@linux.ibm.com,
	sparclinux@vger.kernel.org, davem@davemloft.net,
	catalin.marinas@arm.com, linux-arch@vger.kernel.org,
	linux-kernel@vger.kernel.org, ak@linux.intel.com,
	dave.hansen@intel.com, kirill.shutemov@linux.intel.com
Subject: Re: [PATCH v2 3/6] perf/core: Fix arch_perf_get_page_size()
Date: Thu, 26 Nov 2020 13:27:47 +0000	[thread overview]
Message-ID: <20201126132747.GS4327@casper.infradead.org> (raw)
In-Reply-To: <20201126130619.GI2414@hirez.programming.kicks-ass.net>

On Thu, Nov 26, 2020 at 02:06:19PM +0100, Peter Zijlstra wrote:
> 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.

Right, it's not new.  I wouldn't block this patchset for that fix.
Just want to get the problem on your radar ;-)  I just never reviewed
the gup fast codepath before, and this jumped out at me.

  reply	other threads:[~2020-11-26 13:28 UTC|newest]

Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-26 12:01 [PATCH v2 0/6] perf/mm: Fix PERF_SAMPLE_*_PAGE_SIZE Peter Zijlstra
2020-11-26 12:01 ` [PATCH v2 1/6] mm/gup: Provide gup_get_pte() more generic Peter Zijlstra
2020-11-26 12:43   ` Matthew Wilcox
2020-11-26 13:02     ` Peter Zijlstra
2020-12-03  9:07   ` [tip: perf/core] " tip-bot2 for Peter Zijlstra
2020-12-03  9:24   ` tip-bot2 for Peter Zijlstra
2020-11-26 12:01 ` [PATCH v2 2/6] mm: Introduce pXX_leaf_size() Peter Zijlstra
2020-11-26 12:43   ` Matthew Wilcox
2020-12-03  9:07   ` [tip: perf/core] " tip-bot2 for Peter Zijlstra
2020-12-03  9:24   ` tip-bot2 for Peter Zijlstra
2020-11-26 12:01 ` [PATCH v2 3/6] perf/core: Fix arch_perf_get_page_size() Peter Zijlstra
2020-11-26 12:34   ` Matthew Wilcox
2020-11-26 12:42     ` Peter Zijlstra
2020-11-26 12:56       ` Matthew Wilcox
2020-11-26 13:06         ` Peter Zijlstra
2020-11-26 13:27           ` Matthew Wilcox [this message]
2020-12-03  9:07       ` [tip: perf/core] " tip-bot2 for Peter Zijlstra
2020-12-03  9:24       ` tip-bot2 for Peter Zijlstra
2020-11-26 12:01 ` [PATCH v2 4/6] arm64/mm: Implement pXX_leaf_size() support Peter Zijlstra
2020-11-26 12:57   ` Peter Zijlstra
2020-11-26 14:32     ` Will Deacon
2020-12-03  9:07     ` [tip: perf/core] " tip-bot2 for Peter Zijlstra
2020-12-03  9:24     ` tip-bot2 for Peter Zijlstra
2020-11-26 12:01 ` [PATCH v2 5/6] sparc64/mm: " Peter Zijlstra
2020-12-03  9:07   ` [tip: perf/core] " tip-bot2 for Peter Zijlstra
2020-12-03  9:24   ` tip-bot2 for Peter Zijlstra
2020-12-09 18:44   ` tip-bot2 for Peter Zijlstra
2020-11-26 12:01 ` [PATCH v2 6/6] powerpc/8xx: " Peter Zijlstra
2020-12-03  9:07   ` [tip: perf/core] " tip-bot2 for Peter Zijlstra
2020-12-03  9:24   ` tip-bot2 for Peter Zijlstra
2020-12-09 18:44   ` tip-bot2 for Peter Zijlstra

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