Re: [PATCH v1] mm/ksm: update stale comment in write_protect_page()

[David Hildenbrand <david@redhat.com>]

On 31.08.22 21:34, Yang Shi wrote:
> On Wed, Aug 31, 2022 at 12:15 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 31.08.22 21:08, Yang Shi wrote:
>>> On Wed, Aug 31, 2022 at 11:29 AM David Hildenbrand <david@redhat.com> wrote:
>>>>
>>>> On 31.08.22 19:55, Yang Shi wrote:
>>>>> On Wed, Aug 31, 2022 at 1:30 AM David Hildenbrand <david@redhat.com> wrote:
>>>>>>
>>>>>> The comment is stale, because a TLB flush is no longer sufficient and
>>>>>> required to synchronize against concurrent GUP-fast. This used to be true
>>>>>> in the past, whereby a TLB flush would have implied an IPI on architectures
>>>>>> that support GUP-fast, resulting in GUP-fast that disables local interrupts
>>>>>> from completing before completing the flush.
>>>>>
>>>>> Hmm... it seems there might be problem for THP collapse IIUC. THP
>>>>> collapse clears and flushes pmd before doing anything on pte and
>>>>> relies on interrupt disable of fast GUP to serialize against fast GUP.
>>>>> But if TLB flush is no longer sufficient, then we may run into the
>>>>> below race IIUC:
>>>>>
>>>>>          CPU A                                                CPU B
>>>>> THP collapse                                             fast GUP
>>>>>
>>>>> gup_pmd_range() <-- see valid pmd
>>>>>
>>>>> gup_pte_range() <-- work on pte
>>>>> clear pmd and flush TLB
>>>>> __collapse_huge_page_isolate()
>>>>>     isolate page <-- before GUP bump refcount
>>>>>
>>>>>    pin the page
>>>>> __collapse_huge_page_copy()
>>>>>     copy data to huge page
>>>>>     clear pte (don't flush TLB)
>>>>> Install huge pmd for huge page
>>>>>
>>>>> return the obsolete page
>>>>
>>>> Hm, the is_refcount_suitable() check runs while the PTE hasn't been
>>>> cleared yet. And we don't check if the PMD changed once we're in
>>>> gup_pte_range().
>>>
>>> Yes
>>>
>>>>
>>>> The comment most certainly should be stale as well -- unless there is
>>>> some kind of an implicit IPI broadcast being done.
>>>>
>>>> 2667f50e8b81 mentions: "The RCU page table free logic coupled with an
>>>> IPI broadcast on THP split (which is a rare event), allows one to
>>>> protect a page table walker by merely disabling the interrupts during
>>>> the walk."
>>>>
>>>> I'm not able to quickly locate that IPI broadcast -- maybe there is one
>>>> being done here (in collapse) as well?
>>>
>>> The TLB flush may call IPI. I'm supposed it is arch dependent, right?
>>> Some do use IPI, some may not.
>>
>> Right, and the whole idea of the RCU GUP-fast was to support
>> architectures that don't do it. x86-64 does it. IIRC, powerpc doesn't do
>> it -- but maybe it does so for PMDs?
> 
> It looks powerpc does issue IPI for pmd flush. But arm64 doesn't IIRC.
> 
> So maybe we should implement pmdp_collapse_flush() for those arches to
> issue IPI.

... or find another way to detect and handle this in GUP-fast?

Not sure if, for handling PMDs, it could be sufficient to propagate the
pmdp pointer + value and double check that the values didn't change.

-- 
Thanks,

David / dhildenb