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From: Zi Yan <ziy@nvidia.com>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
CC: <linux-mm@kvack.org>, Roman Gushchin <guro@fb.com>, Rik van Riel
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Subject: Re: [RFC PATCH 01/16] mm: add pagechain container for storing
 multiple pages.
Date: Mon, 7 Sep 2020 11:11:05 -0400
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On 7 Sep 2020, at 8:22, Kirill A. Shutemov wrote:

> On Wed, Sep 02, 2020 at 02:06:13PM -0400, Zi Yan wrote:
>> From: Zi Yan <ziy@nvidia.com>
>>
>> When depositing page table pages for 1GB THPs, we need 512 PTE pages +=

>> 1 PMD page. Instead of counting and depositing 513 pages, we can use t=
he
>> PMD page as a leader page and chain the rest 512 PTE pages with ->lru.=

>> This, however, prevents us depositing PMD pages with ->lru, which is
>> currently used by depositing PTE pages for 2MB THPs. So add a new
>> pagechain container for PMD pages.
>>
>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>
> Just deposit it to a linked list in the mm_struct as we do for PMD if
> split ptl disabled.
>

Thank you for checking the patches. Since we don=E2=80=99t have PUD split=
 lock
yet, I store the PMD page table pages in a newly added linked list head
in mm_struct like you suggested above.

I was too vague about my pagechain design for depositing page table pages=

for PUD THPs. Sorry about the confusion. Let me clarify why
I am doing this pagechain here too. I am sure there would be
some other designs and I am happy to change my code.

In my design, I did not store all page table pages in a single list.
I first deposit 512 PTE pages in one PMD page table page=E2=80=99s pmd_hu=
ge_pte
using pgtable_trans_huge_depsit(), then deposit the PMD page to
a newly added linked list in mm_struct. Since pmd_huge_pte shares space
with half of lru in struct page, we cannot use lru to link all PMD
pages together. As a result, I added pagechain. Also in this way,
we can avoid these things:

1. when we withdraw the PMD page during PUD THP split, we don=E2=80=99t n=
eed
to withdraw 513 page, set up one PMD page, then, deposit 512 PTE pages
in that PMD page.

2. we don=E2=80=99t mix PMD page table pages and PTE page table pages in =
a single
list, since they are initialized in different ways. Otherwise, we need
to maintain a subtle rule in the single page table page list that in ever=
y
513 pages, first one is PMD page table page and the rest are PTE page
table pages.

As I am typing, I also realize that my current design does not work
when PMD split lock is disabled, so I will fix it. I would store PMD page=
s
and PTE pages in two separate lists in mm_struct.


Any comments?


=E2=80=94
Best Regards,
Yan Zi

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