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

[Peter Xu <peterx@redhat.com>]

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