linux-kernel - Re: [PATCH v2 0/2] mm: Remember a/d bits for migration entries

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Date:   Thu, 4 Aug 2022 15:17:44 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Linux MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        Huang Ying <ying.huang@...el.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Hugh Dickins <hughd@...gle.com>,
        Andi Kleen <andi.kleen@...el.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>
Subject: Re: [PATCH v2 0/2] mm: Remember a/d bits for migration entries

On Aug 4, 2022, at 1:39 PM, Peter Xu <peterx@...hat.com> wrote:

>  (1) Page Idle Tracking
> 
>  Before this series, idle tracking can cause false negative if an accessed
>  page got migrated, since after migration the young bit will get lost.
>  After this series, it'll be better in that after migration young bit will
>  be persisted, so it'll be able to be detected correctly by page idle
>  logic when walking the pgtable.
> 
>  However there's still nothing done when page idle reset was carried out
>  during migration procedure in progress, but that should be a separate
>  topic to be addressed (e.g. to teach rmap pgtable walk code to be able to
>  walk with both present ptes and migration ptes).

IIUC, when a migration entry is set page_remove_rmap() is called by
try_to_migrate_one(), so improving page-idle accuracy should be done in a
different way.