linux-kernel - Re: [PATCH v3 5/7] mm: Remember young/dirty bit for page migrations

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Date:   Wed, 10 Aug 2022 14:30:33 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Minchan Kim <minchan@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <andi.kleen@...el.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>
Subject: Re: [PATCH v3 5/7] mm: Remember young/dirty bit for page migrations

Peter Xu <peterx@...hat.com> writes:

> 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 can be big enough to store both the PFN value and the young
                                                                       ~~~~~
Nitpick: A/D

> bit.  Otherwise the A/D bits are dropped like before.
>
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>  include/linux/swapops.h | 99 +++++++++++++++++++++++++++++++++++++++++
>  mm/huge_memory.c        | 18 +++++++-
>  mm/migrate.c            |  6 ++-
>  mm/migrate_device.c     |  4 ++
>  mm/rmap.c               |  5 ++-
>  5 files changed, 128 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> index e1accbcd1136..0e9579b90659 100644
> --- a/include/linux/swapops.h
> +++ b/include/linux/swapops.h
> @@ -8,6 +8,10 @@
>  
>  #ifdef CONFIG_MMU
>  
> +#ifdef CONFIG_SWAP
> +#include <linux/swapfile.h>
> +#endif	/* CONFIG_SWAP */

I don't think we need the comment here.  The #ifdef is too near.  But
this isn't a big deal.

Best Regards,
Huang, Ying