From: David Hildenbrand <david@redhat.com>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>,
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 16:44:20 +0200 [thread overview]
Message-ID: <40486dbb-9f19-6fa6-d46d-99d2b033883d@redhat.com> (raw)
In-Reply-To: <YxdZlCly2ad1rtcI@nvidia.com>
On 06.09.22 16:30, Jason Gunthorpe wrote:
> On Tue, Sep 06, 2022 at 03:57:30PM +0200, David Hildenbrand wrote:
>
>>> READ_ONCE primarily is a marker that the data being read is unstable
>>> and that the compiler must avoid all instability when reading it. eg
>>> in this case the compiler could insanely double read the value, even
>>> though the 'if' requires only a single read. This would result in
>>> corrupt calculation.
>>
>> As we have a full memory barrier + compile barrier, the compiler might
>> indeed do double reads and all that stuff. BUT, it has to re-read after we
>> incremented the refcount, and IMHO that's the important part to detect the
>> change.
>
> Yes, it is important, but it is not the only important part.
>
> The compiler still has to exectute "if (*a != b)" *correctly*.
>
> This is what READ_ONCE is for. It doesn't set order, it doesn't
> implement a barrier, it tells the compiler that '*a' is unstable data
> and the compiler cannot make assumptions based on the idea that
> reading '*a' multiple times will always return the same value.
>
> If the compiler makes those assumptions then maybe even though 'if (*a
> != b)' is the reality, it could mis-compute '*a == b'. You enter into
> undefined behavior here.
>
> 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.
... I do wonder if we want to reuse ptep_get_lockless() instead of the
READ_ONCE(). CONFIG_GUP_GET_PTE_LOW_HIGH is confusing.
--
Thanks,
David / dhildenb
next prev parent reply other threads:[~2022-09-06 15:35 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 [this message]
2022-09-06 15:33 ` Jason Gunthorpe
2022-09-06 19:11 ` Yang Shi
2022-09-06 23:16 ` John Hubbard
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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