Re: [PATCH v1] mm: Fix race between __split_huge_pmd_locked() and GUP-fast

[Ryan Roberts <ryan.roberts@arm.com>]

On 29/04/2024 16:29, Zi Yan wrote:
> On 29 Apr 2024, at 10:45, Zi Yan wrote:
> 
>> On 29 Apr 2024, at 5:29, Ryan Roberts wrote:
>>
>>> On 27/04/2024 20:11, John Hubbard wrote:
>>>> On 4/27/24 8:14 AM, Zi Yan wrote:
>>>>> On 27 Apr 2024, at 0:41, John Hubbard wrote:
>>>>>> On 4/25/24 10:07 AM, Ryan Roberts wrote:
>>>>>>> __split_huge_pmd_locked() can be called for a present THP, devmap or
>>>>>>> (non-present) migration entry. It calls pmdp_invalidate()
>>>>>>> unconditionally on the pmdp and only determines if it is present or not
>>>>>>> based on the returned old pmd. This is a problem for the migration entry
>>>>>>> case because pmd_mkinvalid(), called by pmdp_invalidate() must only be
>>>>>>> called for a present pmd.
>>>>>>>
>>>>>>> On arm64 at least, pmd_mkinvalid() will mark the pmd such that any
>>>>>>> future call to pmd_present() will return true. And therefore any
>>>>>>> lockless pgtable walker could see the migration entry pmd in this state
>>>>>>> and start interpretting the fields as if it were present, leading to
>>>>>>> BadThings (TM). GUP-fast appears to be one such lockless pgtable walker.
>>>>>>> I suspect the same is possible on other architectures.
>>>>>>>
>>>>>>> Fix this by only calling pmdp_invalidate() for a present pmd. And for
>>>>>>
>>>>>> Yes, this seems like a good design decision (after reading through the
>>>>>> discussion that you all had in the other threads).
>>>>>
>>>>> This will only be good for arm64 and does not prevent other arch developers
>>>>> to write code breaking arm64, since only arm64's pmd_mkinvalid() can turn
>>>>> a swap entry to a pmd_present() entry.
>>>>
>>>> Well, let's characterize it in a bit more detail, then:
>>>
>>> Hi All,
>>>
>>> Thanks for all the feedback! I had thought that this patch would be entirely
>>> uncontraversial - obviously I was wrong :)
>>>
>>> I've read all the emails, and trying to summarize a way forward here...
>>>
>>>>
>>>> 1) This patch will make things better for arm64. That's important!
>>>>
>>>> 2) Equally important, this patch does not make anything worse for
>>>>    other CPU arches.
>>>>
>>>> 3) This patch represents a new design constraint on the CPU arch
>>>>    layer, and thus requires documentation and whatever enforcement
>>>>    we can provide, in order to keep future code out of trouble.
>>>
>>> I know its only semantics, but I don't view this as a new design constraint. I
>>> see it as an existing constraint that was previously being violated, and this
>>> patch aims to fix that. The generic version of pmdp_invalidate() unconditionally
>>> does a tlb invalidation on the address range covered by the pmd. That makes no
>>> sense unless the pmd was previously present. So my conclusion is that the
>>> function only expects to be called for present pmds.
>>>
>>> Additionally Documentation/mm/arch_pgtable_helpers.rst already says this:
>>>
>>> "
>>> | pmd_mkinvalid             | Invalidates a mapped PMD [1]                     |
>>> "
>>>
>>> I read "mapped" to be a synonym for "present". So I think its already
>>> documented. Happy to explcitly change "mapped" to "present" though, if it helps?
>>>
>>> Finally, [1] which is linked from Documentation/mm/arch_pgtable_helpers.rst,
>>> also implies this constraint, although it doesn't explicitly say it.
>>>
>>> [1] https://lore.kernel.org/linux-mm/20181017020930.GN30832@redhat.com/
>>>
>>>>
>>>> 3.a) See the VM_WARN_ON() hunks below.
>>>
>>> It sounds like everybody would be happy if I sprinkle these into the arches that
>>> override pmdp_invalidate[_ad]()? There are 3 arches that have their own version
>>> of pmdp_invalidate(); powerpc, s390 and sparc. And 1 that has its own version of
>>> pmdp_invalidate_ad(); x86. I'll add them in all of those.
>>>
>>> I'll use VM_WARN_ON_ONCE() as suggested by John.
>>>
>>> I'd rather not put it directly into pmd_mkinvalid() since that would set a
>>> precedent for adding them absolutely everywhere. (e.g. pte_mkdirty(), ...).
>>
>> I understand your concern here. I assume you also understand the potential issue
>> with this, namely it does not prevent one from using pmd_mkinvalid() improperly
>> and causing a bug and the bug might only appear on arm64.
>>
>>>
>>>>
>>>> 3.b) I like the new design constraint, because it is reasonable and
>>>>      clearly understandable: don't invalidate a non-present page
>>>>      table entry.
>>>>
>>>> I do wonder if there is somewhere else that this should be documented?
>>>
>>> If I change:
>>>
>>> "
>>> | pmd_mkinvalid             | Invalidates a mapped PMD [1]                     |
>>> "
>>>
>>> To:
>>>
>>> "
>>> | pmd_mkinvalid             | Invalidates a present PMD; do not call for       |
>>> |                             non-present pmd [1]                              |
>>> "
>>>
>>> Is that sufficient? (I'll do the same for pud_mkinvalid() too.
>>
>> Sounds good to me.
>>
>> Also, if you move pmdp_invalidate(), please move the big comment with it to
>> avoid confusion. Thanks.
> 
> And the Fixes tag does not need to go back that far, since this only affects arm64,
> which enables thp migration at commit 53fa117bb33c ("arm64/mm: Enable THP migration").

Yes, will do - good point.

> 
> --
> Best Regards,
> Yan, Zi