Re: [RFC PATCH v4 2/6] mm/migrate: skip data copy for already-copied folios

["David Hildenbrand (Arm)" <david@kernel.org>]

On 3/9/26 13:07, Shivank Garg wrote:
> Add a PAGE_ALREADY_COPIED flag to the dst->private migration state.
> When set, __migrate_folio() skips folio_mc_copy() and performs
> metadata-only migration. All callers currently pass
> already_copied=false. The batch-copy path enables it in a later patch.
> 
> Move the dst->private state enum earlier in the file so
> __migrate_folio() and move_to_new_folio() can see PAGE_ALREADY_COPIED.
> 
> Signed-off-by: Shivank Garg <shivankg@amd.com>
> ---
>  mm/migrate.c | 52 +++++++++++++++++++++++++++++++---------------------
>  1 file changed, 31 insertions(+), 21 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 1bf2cf8c44dd..1d8c1fb627c9 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -848,6 +848,18 @@ void folio_migrate_flags(struct folio *newfolio, struct folio *folio)
>  }
>  EXPORT_SYMBOL(folio_migrate_flags);
>  
> +/*
> + * To record some information during migration, we use unused private
> + * field of struct folio of the newly allocated destination folio.
> + * This is safe because nobody is using it except us.
> + */
> +enum {
> +	PAGE_WAS_MAPPED = BIT(0),
> +	PAGE_WAS_MLOCKED = BIT(1),
> +	PAGE_ALREADY_COPIED = BIT(2),
> +	PAGE_OLD_STATES = PAGE_WAS_MAPPED | PAGE_WAS_MLOCKED | PAGE_ALREADY_COPIED,

All these states really only apply to proper folios (not movable_ops).
So once we complete decoupling movable_ops migration from folio
migration, these flags would only appear in the folio migration part.

Can we convert them first to state it clearly already that these are
folio migration flags?

FOLIO_MF_WAS_MAPPED

...

-- 
Cheers,

David