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

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Date:   Thu, 3 Sep 2020 04:15:45 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Zi Yan <ziy@...dia.com>
Cc:     linux-mm@...ck.org, Roman Gushchin <guro@...com>,
        Rik van Riel <riel@...riel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        David Nellans <dnellans@...dia.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 01/16] mm: add pagechain container for storing
 multiple pages.

On Wed, Sep 02, 2020 at 02:06:13PM -0400, Zi Yan wrote:
> 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.

But you've allocated a page for the PMD table.  Why can't you use that
4kB to store pointers to the 512 PTE tables?

You could also use an existing data structure like the XArray (although
not a pagevec).