linux-kernel - Re: [PATCH v4 5/7] mm: Remember young/dirty bit for page migrations

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Date:   Tue, 13 Sep 2022 08:55:35 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Peter Xu <peterx@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Hugh Dickins <hughd@...gle.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Alistair Popple <apopple@...dia.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Minchan Kim <minchan@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Andi Kleen <andi.kleen@...el.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH v4 5/7] mm: Remember young/dirty bit for page migrations

Andrew Morton <akpm@...ux-foundation.org> writes:

> On Thu, 11 Aug 2022 12:13:29 -0400 Peter Xu <peterx@...hat.com> wrote:
>
>> When page migration happens, we always ignore the young/dirty bit settings
>> in the old pgtable, and marking the page as old in the new page table using
>> either pte_mkold() or pmd_mkold(), and keeping the pte clean.
>> 
>> That's fine from functional-wise, but that's not friendly to page reclaim
>> because the moving page can be actively accessed within the procedure.  Not
>> to mention hardware setting the young bit can bring quite some overhead on
>> some systems, e.g. x86_64 needs a few hundreds nanoseconds to set the bit.
>> The same slowdown problem to dirty bits when the memory is first written
>> after page migration happened.
>> 
>> Actually we can easily remember the A/D bit configuration and recover the
>> information after the page is migrated.  To achieve it, define a new set of
>> bits in the migration swap offset field to cache the A/D bits for old pte.
>> Then when removing/recovering the migration entry, we can recover the A/D
>> bits even if the page changed.
>> 
>> One thing to mention is that here we used max_swapfile_size() to detect how
>> many swp offset bits we have, and we'll only enable this feature if we know
>> the swp offset is big enough to store both the PFN value and the A/D bits.
>> Otherwise the A/D bits are dropped like before.
>> 
>
> There was some discussion over v3 of this patch, but none over v4.
>
> Can people please review this patch series so we can get moving with it?

Most discussions over v3 are for migrate_device.c code.  There are some
bugs and they have been fixed by Alistair via [1].

This patch itself is good.  Sorry for bothering.

Reviewed-by: "Huang, Ying" <ying.huang@...el.com>

[1] https://lore.kernel.org/linux-mm/9f801e9d8d830408f2ca27821f606e09aa856899.1662078528.git-series.apopple@nvidia.com/

Best Regards,
Huang, Ying