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

[Zi Yan]

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@xxxxxxxxxx/
>
>>
>> 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.

--
Best Regards,
Yan, Zi

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