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

From: Kirill A. Shutemov
Date: Wed Sep 09 2020 - 12:43:59 EST

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?

Kirill A. Shutemov