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Date:   Thu, 11 Aug 2022 11:19:35 -0400
From:   Peter Xu <peterx@redhat.com>
To:     linux-mm@kvack.org, linux-kernel@vger.kernel.org
Cc:     Minchan Kim <minchan@kernel.org>,
        David Hildenbrand <david@redhat.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andi Kleen <andi.kleen@intel.com>,
        Huang Ying <ying.huang@intel.com>,
        "Kirill A . Shutemov" <kirill@shutemov.name>
Subject: Re: [PATCH v3 5/7] mm: Remember young/dirty bit for page migrations
Message-ID: <YvUeB0jc6clz59z5@xz-m1.local>
References: <20220809220100.20033-1-peterx@redhat.com>
 <20220809220100.20033-6-peterx@redhat.com>
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On Tue, Aug 09, 2022 at 06:00:58PM -0400, Peter Xu wrote:
> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> index 27fb37d65476..699f821b8443 100644
> --- a/mm/migrate_device.c
> +++ b/mm/migrate_device.c
> @@ -221,6 +221,10 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>  			else
>  				entry = make_readable_migration_entry(
>  							page_to_pfn(page));
> +			if (pte_young(pte))
> +				entry = make_migration_entry_young(entry);
> +			if (pte_dirty(pte))
> +				entry = make_migration_entry_dirty(entry);
>  			swp_pte = swp_entry_to_pte(entry);
>  			if (pte_present(pte)) {
>  				if (pte_soft_dirty(pte))

This change needs to be wrapped with pte_present() at least..

I also just noticed that this change probably won't help anyway because:

  (1) When ram->device, the pte will finally be replaced with a device
      private entry, and device private entry does not yet support A/D, it
      means A/D will be dropped again,

  (2) When device->ram, we are missing information on either A/D bits, or
      even if device private entries start to suport A/D, it's still not
      clear whether we should take device read/write into considerations
      too on the page A/D bits to be accurate.

I think I'll probably keep the code there for completeness, but I think it
won't really help much until more things are done.

-- 
Peter Xu