From: John Hubbard <jhubbard@nvidia.com>
To: David Hildenbrand <david@redhat.com>, Jason Gunthorpe <jgg@nvidia.com>
Cc: Yang Shi <shy828301@gmail.com>,
peterx@redhat.com, kirill.shutemov@linux.intel.com,
hughd@google.com, akpm@linux-foundation.org, linux-mm@kvack.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: gup: fix the fast GUP race against THP collapse
Date: Tue, 6 Sep 2022 16:16:42 -0700 [thread overview]
Message-ID: <778f60b7-97c7-3c35-1b40-40cf844fe89e@nvidia.com> (raw)
In-Reply-To: <40486dbb-9f19-6fa6-d46d-99d2b033883d@redhat.com>
On 9/6/22 07:44, David Hildenbrand wrote:
>> Though it is all very unlikely, the general memory model standard is
>> to annotate with READ_ONCE.
>
> The only thing I could see going wrong in the comparison once the stars
> alingn would be something like the following:
>
> if (*a != b)
>
> implemented as
>
> if ((*a).lower != b.lower && (*a).higher != b.higher)
>
>
> This could only go wrong if we have more than one change such that:
>
> Original:
>
> *a = 0x00000000ffffffffull;
>
>
> First modification:
> *a = 0xffffffffffffffffull;
>
> Second modification:
> *a = 0x00000000eeeeeeeeull;
>
>
> If we race with both modifications, we could see that ffffffff matches,
> and could see that 00000000 matches as well.
>
>
> So I agree that we should change it, but not necessarily as an urgent
> fix and not necessarily in this patch. It's best to adjust all gup_*
> functions in one patch.
>
We had a long thread with Paul McKenney back in May [1] about this exact
sort of problem.
In that thread, I recall that "someone" tried to claim that a bare
one-byte read was safe, and even that innocent-sounding claim got
basically torn apart! :) Because the kernel memory model simply does
not cover you for bare reads and writes to shared mutable memory.
Unfortunately, until now, I'd only really remembered the conclusion:
"use READ_ONCE() and WRITE_ONCE() for any touching of shared mutable
memory", and not the point about other memory barriers not covering this
aspect. Thanks to Jason for reminding us of this. This time I think I
will remember it well enough to avoid another long thread. Maybe.
[1] https://lore.kernel.org/all/20220524163728.GO1790663@paulmck-ThinkPad-P17-Gen-1/
thanks,
--
John Hubbard
NVIDIA
next prev parent reply other threads:[~2022-09-06 23:16 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-01 22:27 [PATCH] mm: gup: fix the fast GUP race against THP collapse Yang Shi
2022-09-01 23:26 ` Peter Xu
2022-09-01 23:50 ` Yang Shi
2022-09-02 6:39 ` David Hildenbrand
2022-09-02 15:23 ` Yang Shi
2022-09-02 15:59 ` Peter Xu
2022-09-02 16:04 ` Peter Xu
2022-09-02 17:30 ` Yang Shi
2022-09-02 17:45 ` Yang Shi
2022-09-02 20:33 ` Peter Xu
2022-09-05 8:56 ` Aneesh Kumar K.V
2022-09-05 8:54 ` Aneesh Kumar K.V
2022-09-06 19:07 ` Yang Shi
2022-09-07 4:50 ` Aneesh Kumar K V
2022-09-07 17:08 ` Yang Shi
2022-09-04 22:21 ` John Hubbard
2022-09-02 6:42 ` David Hildenbrand
2022-09-04 22:29 ` John Hubbard
2022-09-05 7:59 ` David Hildenbrand
2022-09-05 10:16 ` Baolin Wang
2022-09-05 10:24 ` David Hildenbrand
2022-09-05 11:11 ` David Hildenbrand
2022-09-05 14:35 ` Baolin Wang
2022-09-05 14:40 ` David Hildenbrand
2022-09-06 5:53 ` Baolin Wang
2022-09-06 2:12 ` John Hubbard
2022-09-06 12:50 ` David Hildenbrand
2022-09-06 13:47 ` Jason Gunthorpe
2022-09-06 13:57 ` David Hildenbrand
2022-09-06 14:30 ` Jason Gunthorpe
2022-09-06 14:44 ` David Hildenbrand
2022-09-06 15:33 ` Jason Gunthorpe
2022-09-06 19:11 ` Yang Shi
2022-09-06 23:16 ` John Hubbard [this message]
2022-09-06 19:01 ` Yang Shi
2022-09-05 9:03 ` Baolin Wang
2022-09-06 18:50 ` Yang Shi
2022-09-06 21:27 ` John Hubbard
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