Re: [RFC PATCH 01/16] mm: add pagechain container for storing multiple pages.

[Zi Yan]

On 9 Sep 2020, at 9:46, Kirill A. Shutemov wrote:

> On Mon, Sep 07, 2020 at 11:11:05AM -0400, Zi Yan wrote:
>> 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@xxxxxxxxxx>
>>>>
>>>> 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 the
>>>> 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@xxxxxxxxxx>
>>>
>>> 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’t 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’s pmd_huge_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’t need
>> to withdraw 513 page, set up one PMD page, then, deposit 512 PTE pages
>> in that PMD page.
>>
>> 2. we don’t 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 every
>> 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 pages
>> and PTE pages in two separate lists in mm_struct.
>>
>>
>> Any comments?
>
> Okay, fair enough.
>
> Although, I think you can get away without a new data structure. We don't
> need double-linked list to deposit page tables. You can rework PTE tables
> deposit code to have single-linked list and use one pointer of ->lru (with
> proper name) and make PMD tables deposit to use the other one. This way
> you can avoid conflict for ->lru.
>
> Does it make sense?

Yes. Thanks. Will do this in the next version. I think the single linked list
from llist.h can be used.

—
Best Regards,
Yan Zi

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