Message-ID: <5E6C3511-EEB5-4DE7-8838-281E3053A0C7@nvidia.com>
Date: Thu, 09 May 2024 10:53:36 -0400
From: Zi Yan <ziy@...dia.com>
To: Lance Yang <ioworker0@...il.com>
Cc: Jason Gunthorpe <jgg@...dia.com>, Alistair Popple <apopple@...dia.com>,
 akpm@...ux-foundation.org, willy@...radead.org, sj@...nel.org,
 maskray@...gle.com, ryan.roberts@....com, david@...hat.com,
 21cnbao@...il.com, mhocko@...e.com, fengwei.yin@...el.com,
 zokeefe@...gle.com, shy828301@...il.com, xiehuan09@...il.com,
 libang.li@...group.com, wangkefeng.wang@...wei.com, songmuchun@...edance.com,
 peterx@...hat.com, minchan@...nel.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Baolin Wang <baolin.wang@...ux.alibaba.com>
Subject: Re: [PATCH v4 2/3] mm/rmap: integrate PMD-mapped folio splitting into
 pagewalk loop

On 9 May 2024, at 4:21, Lance Yang wrote:

> Hey Zi and Jason,
>
> Thanks a lot for reaching out!
>
> On Thu, May 9, 2024 at 12:35 AM Jason Gunthorpe <jgg@...dia.com> wrote:
>>
>> On Wed, May 08, 2024 at 12:22:08PM -0400, Zi Yan wrote:
>>> On 8 May 2024, at 11:52, Jason Gunthorpe wrote:
>>>
>>>> On Wed, May 08, 2024 at 10:56:34AM -0400, Zi Yan wrote:
>>>>
>>>>> Lance is improving try_to_unmap_one() to support unmapping PMD THP as a whole,
>>>>> so he moves split_huge_pmd_address() inside while (page_vma_mapped_walk(&pvmw))
>>>>> and after mmu_notifier_invalidate_range_start() as split_huge_pmd_locked()
>>>>> and does not include the mmu notifier ops inside split_huge_pmd_address().
>
> IMO, It might be reasonable to exclude the mmu notifier ops in
> split_huge_pmd_locked(). IIUC, before acquiring the PTL, callers need to tear
> down the secondary mappings via mmu_notifier_invalidate_range_start() with
> the range aligned to HPAGE_PMD_SIZE.
>
>>>>> I wonder if that could cause issues, since the mmu_notifier_invalidate_range_start()
>>>>> before the while loop only has range of the original address and
>>>>> split huge pmd can affect the entire PMD address range and these two ranges
>>>>> might not be the same.
>
> As Baolin mentioned [1] before:
> "For a PMD mapped THP, I think the address is already THP size alignment
> returned from vma_address(&folio->page, vma)."
>
> Given this, perhaps we don't need to re-align the input address after
> starting the pagewalk? IMO, if any corner cases arise, we should catch them
> by using VM_WARN_ON_ONCE() in split_huge_pmd_locked().
>
> Zi, what do you think?

Yes, I agree. Thanks for sorting this out.

>
> [1] https://lore.kernel.org/linux-mm/cc9fd23f-7d87-48a7-a737-acbea8e95fb7@linux.alibaba.com/
>
>>>>
>>>> That does not sound entirely good..
>>>>
>>>> I suppose it depends on what split does, if the MM page table has the
>>>> same translation before and after split then perhaps no invalidation
>>>> is even necessary.
>>>
>>> Before split, it is a PMD mapping to a PMD THP (order-9). After split,
>>> they are 512 PTEs mapping to the same THP. Unless the secondary TLB
>>> does not support PMD mapping and use 512 PTEs instead, it seems to
>>> be an issue from my understanding.
>>
>> I may not recall fully, but I don't think any secondaries are
>> so sensitive to the PMD/PTE distinction.. At least the ones using
>> hmm_range_fault() are not.
>>
>> When the PTE eventually comes up for invalidation then the secondary
>> should wipe out any granual they may have captured.
>>
>> Though, perhaps KVM should be checked carefully.
>>
>>> In terms of two mmu_notifier ranges, first is in the split_huge_pmd_address()[1]
>>> and second is in try_to_unmap_one()[2]. When try_to_unmap_one() is unmapping
>>> a subpage in the middle of a PMD THP, the former notifies about the PMD range
>>> change due to one PMD split into 512 PTEs and the latter only needs to notify
>>> about the invalidation of the unmapped PTE. I do not think the latter can
>>> replace the former, although a potential optimization can be that the latter
>>> can be removed as it is included in the range of the former.
>>
>> I think we probably don't need both, either size might be fine, but
>> the larger size is definately fine..
>>
>>> Regarding Lance's current code change, is it OK to change mmu_notifier range
>>> after mmu_notifier_invalidate_range_start()?
>>
>> No, it cannot be changed during a start/stop transaction.
>
> I understood and will keep that in mind - thanks!
>
> Thanks again for clarifying!
> Lance
>
>>
>> Jason
>>
>>


--
Best Regards,
Yan, Zi

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