From: Zi Yan <ziy@nvidia.com>
To: Lance Yang <ioworker0@gmail.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>,
Alistair Popple <apopple@nvidia.com>,
akpm@linux-foundation.org, willy@infradead.org, sj@kernel.org,
maskray@google.com, ryan.roberts@arm.com, david@redhat.com,
21cnbao@gmail.com, mhocko@suse.com, fengwei.yin@intel.com,
zokeefe@google.com, shy828301@gmail.com, xiehuan09@gmail.com,
libang.li@antgroup.com, wangkefeng.wang@huawei.com,
songmuchun@bytedance.com, peterx@redhat.com, minchan@kernel.org,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Baolin Wang <baolin.wang@linux.alibaba.com>
Subject: Re: [PATCH v4 2/3] mm/rmap: integrate PMD-mapped folio splitting into pagewalk loop
Date: Thu, 09 May 2024 10:53:36 -0400 [thread overview]
Message-ID: <5E6C3511-EEB5-4DE7-8838-281E3053A0C7@nvidia.com> (raw)
In-Reply-To: <CAK1f24=PmS1RMQ6=0dgX_q9xqUthWOjJBz_FE-Ndb7MwGLWYdg@mail.gmail.com>
[-- Attachment #1: Type: text/plain, Size: 3712 bytes --]
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@nvidia.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
[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 854 bytes --]
next prev parent reply other threads:[~2024-05-09 14:53 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-05-01 4:26 [PATCH v4 0/3] Reclaim lazyfree THP without splitting Lance Yang
2024-05-01 4:26 ` [PATCH v4 1/3] mm/rmap: remove duplicated exit code in pagewalk loop Lance Yang
2024-05-07 14:51 ` Zi Yan
2024-05-07 14:53 ` Lance Yang
2024-05-01 4:26 ` [PATCH v4 2/3] mm/rmap: integrate PMD-mapped folio splitting into " Lance Yang
2024-05-07 3:40 ` Baolin Wang
2024-05-07 4:37 ` Lance Yang
2024-05-07 8:17 ` David Hildenbrand
2024-05-07 8:38 ` Lance Yang
2024-05-07 17:22 ` Andrew Morton
2024-05-07 17:33 ` David Hildenbrand
2024-05-07 17:38 ` Andrew Morton
2024-05-07 18:30 ` David Hildenbrand
2024-05-07 15:26 ` Zi Yan
2024-05-08 5:43 ` Lance Yang
2024-05-08 14:07 ` Zi Yan
2024-05-08 14:35 ` Lance Yang
2024-05-08 14:48 ` Zi Yan
2024-05-08 14:56 ` Zi Yan
2024-05-08 15:52 ` Jason Gunthorpe
2024-05-08 16:22 ` Zi Yan
2024-05-08 16:35 ` Jason Gunthorpe
2024-05-09 8:21 ` Lance Yang
2024-05-09 14:53 ` Zi Yan [this message]
2024-05-09 8:56 ` Lance Yang
2024-05-01 4:27 ` [PATCH v4 3/3] mm/vmscan: avoid split lazyfree THP during shrink_folio_list() Lance Yang
2024-05-07 4:00 ` Baolin Wang
2024-05-07 6:32 ` Lance Yang
2024-05-07 8:26 ` Lance Yang
2024-05-07 9:33 ` Baolin Wang
2024-05-07 11:37 ` Lance Yang
2024-05-09 9:36 ` Baolin Wang
2024-05-09 12:17 ` Lance Yang
2024-05-07 16:20 ` Zi Yan
2024-05-08 5:14 ` Lance Yang
2024-05-01 16:08 ` [PATCH v4 0/3] Reclaim lazyfree THP without splitting SeongJae Park
2024-05-02 0:30 ` Lance Yang
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=5E6C3511-EEB5-4DE7-8838-281E3053A0C7@nvidia.com \
--to=ziy@nvidia.com \
--cc=21cnbao@gmail.com \
--cc=akpm@linux-foundation.org \
--cc=apopple@nvidia.com \
--cc=baolin.wang@linux.alibaba.com \
--cc=david@redhat.com \
--cc=fengwei.yin@intel.com \
--cc=ioworker0@gmail.com \
--cc=jgg@nvidia.com \
--cc=libang.li@antgroup.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=maskray@google.com \
--cc=mhocko@suse.com \
--cc=minchan@kernel.org \
--cc=peterx@redhat.com \
--cc=ryan.roberts@arm.com \
--cc=shy828301@gmail.com \
--cc=sj@kernel.org \
--cc=songmuchun@bytedance.com \
--cc=wangkefeng.wang@huawei.com \
--cc=willy@infradead.org \
--cc=xiehuan09@gmail.com \
--cc=zokeefe@google.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).