linux-kernel - Re: [PATCH v1 06/14] mm, x86: support the access bit on non-leaf PMD entries

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Date:   Sun, 14 Mar 2021 20:27:29 -0400
From:   "Zi Yan" <ziy@...dia.com>
To:     "Yu Zhao" <yuzhao@...gle.com>,
        "Matthew Wilcox" <willy@...radead.org>
Cc:     linux-mm@...ck.org, "Alex Shi" <alex.shi@...ux.alibaba.com>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Dave Hansen" <dave.hansen@...ux.intel.com>,
        "Hillf Danton" <hdanton@...a.com>,
        "Johannes Weiner" <hannes@...xchg.org>,
        "Joonsoo Kim" <iamjoonsoo.kim@....com>,
        "Mel Gorman" <mgorman@...e.de>, "Michal Hocko" <mhocko@...e.com>,
        "Roman Gushchin" <guro@...com>, "Vlastimil Babka" <vbabka@...e.cz>,
        "Wei Yang" <richard.weiyang@...ux.alibaba.com>,
        "Yang Shi" <shy828301@...il.com>,
        "Ying Huang" <ying.huang@...el.com>, linux-kernel@...r.kernel.org,
        page-reclaim@...gle.com
Subject: Re: [PATCH v1 06/14] mm, x86: support the access bit on non-leaf PMD
 entries

On 14 Mar 2021, at 20:03, Yu Zhao wrote:

> On Sun, Mar 14, 2021 at 10:51:03PM +0000, Matthew Wilcox wrote:
>> On Sun, Mar 14, 2021 at 06:12:42PM -0400, Zi Yan wrote:
>>> On 13 Mar 2021, at 2:57, Yu Zhao wrote:
>>>
>>>> Some architectures support the accessed bit on non-leaf PMD entries
>>>> (parents) in addition to leaf PTE entries (children) where pages are
>>>> mapped, e.g., x86_64 sets the accessed bit on a parent when using it
>>>> as part of linear-address translation [1]. Page table walkers who are
>>>> interested in the accessed bit on children can take advantage of this:
>>>> they do not need to search the children when the accessed bit is not
>>>> set on a parent, given that they have previously cleared the accessed
>>>> bit on this parent in addition to its children.
>>>>
>>>> [1]: Intel 64 and IA-32 Architectures Software Developer's Manual
>>>>      Volume 3 (October 2019), section 4.8
>>>
>>> Just curious. Does this also apply to non-leaf PUD entries? Do you
>>> mind sharing which sentence from the manual gives the information?
>>
>> The first few sentences from 4.8:
>>
>> : For any paging-structure entry that is used during linear-address
>> : translation, bit 5 is the accessed flag. For paging-structure
>> : entries that map a page (as opposed to referencing another paging
>> : structure), bit 6 is the dirty flag. These flags are provided for
>> : use by memory-management software to manage the transfer of pages and
>> : paging structures into and out of physical memory.
>>
>> : Whenever the processor uses a paging-structure entry as part of
>> : linear-address translation, it sets the accessed flag in that entry
>> : (if it is not already set).

Matthew, thanks for the pointer.

>
> As far as I know x86 is the one that supports this.
>
>> The way they differentiate between the A and D bits makes it clear to
>> me that the A bit is set at each level of the tree, but the D bit is
>> only set on leaf entries.
>
> And the difference makes perfect sense (to me). Kudos to Intel.

Hi Yu,

You only introduced HAVE_ARCH_PARENT_PMD_YOUNG but no HAVE_ARCH_PARENT_PUD_YOUNG.
Is it PUD granularity too large to be useful for multigenerational LRU algorithm?

Thanks.

—
Best Regards,
Yan Zi

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