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

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Date:   Wed, 2 Sep 2020 16:48:59 -0400
From:   Zi Yan <ziy@...dia.com>
To:     Randy Dunlap <rdunlap@...radead.org>
CC:     <linux-mm@...ck.org>, Roman Gushchin <guro@...com>,
        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 2 Sep 2020, at 16:29, Randy Dunlap wrote:

> 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 Is the comment out of date or am I just confuzed?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?

My bad. Will change this to:

/* 15 pointers + one long align the pagechain structure to a power of two */
#define PAGECHAIN_SIZE  13

struct page;

struct pagechain {
    struct list_head list;
    unsigned long nr;
    struct page *pages[PAGECHAIN_SIZE];
};


Thanks for checking.

—
Best Regards,
Yan Zi

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