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

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Message-ID: <5a91ac07-2c15-2e15-caec-5570bacf415b@infradead.org>
Date:   Wed, 2 Sep 2020 13:29:33 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Zi Yan <ziy@...dia.com>, linux-mm@...ck.org,
        Roman Gushchin <guro@...com>
Cc:     Rik van Riel <riel@...riel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        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 9/2/20 11:06 AM, Zi Yan wrote:
> From: Zi Yan <ziy@...dia.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 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@...dia.com>
> ---
>  include/linux/pagechain.h | 73 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 73 insertions(+)
>  create mode 100644 include/linux/pagechain.h
> 
> diff --git a/include/linux/pagechain.h b/include/linux/pagechain.h
> new file mode 100644
> index 000000000000..be536142b413
> --- /dev/null
> +++ b/include/linux/pagechain.h
> @@ -0,0 +1,73 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * include/linux/pagechain.h
> + *
> + * In many places it is efficient to batch an operation up against multiple
> + * pages. A pagechain is a multipage container which is used for that.
> + */
> +
> +#ifndef _LINUX_PAGECHAIN_H
> +#define _LINUX_PAGECHAIN_H
> +
> +#include <linux/slab.h>
> +
> +/* 14 pointers + two long's align the pagechain structure to a power of two */
> +#define PAGECHAIN_SIZE	13

OK, I'll bite.  I see neither 14 pointers nor 2 longs below.
Is the comment out of date or am I just confuzed?

Update: struct list_head is 2 pointers, so I see 15 pointers & one unsigned int.
Where are the 2 longs?

> +
> +struct page;
> +
> +struct pagechain {
> +	struct list_head list;
> +	unsigned int nr;
> +	struct page *pages[PAGECHAIN_SIZE];
> +};

thanks.

-- 
~Randy