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

["Huang, Ying" <ying.huang@linux.alibaba.com>]

Shivank Garg <shivankg@amd.com> writes:

> 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,
> +};
> +
>  /************************************************************
>   *                    Migration functions
>   ***********************************************************/
> @@ -857,14 +869,20 @@ static int __migrate_folio(struct address_space *mapping, struct folio *dst,
>  			   enum migrate_mode mode)
>  {
>  	int rc, expected_count = folio_expected_ref_count(src) + 1;
> +	bool already_copied = ((unsigned long)dst->private & PAGE_ALREADY_COPIED);
> +
> +	if (already_copied)
> +		dst->private = NULL;
>  
>  	/* Check whether src does not have extra refs before we do more work */
>  	if (folio_ref_count(src) != expected_count)
>  		return -EAGAIN;
>  
> -	rc = folio_mc_copy(dst, src);
> -	if (unlikely(rc))
> -		return rc;
> +	if (!already_copied) {
> +		rc = folio_mc_copy(dst, src);
> +		if (unlikely(rc))
> +			return rc;
> +	}
>  
>  	rc = __folio_migrate_mapping(mapping, dst, src, expected_count);
>  	if (rc)
> @@ -1088,7 +1106,7 @@ static int fallback_migrate_folio(struct address_space *mapping,
>   *     0 - success
>   */
>  static int move_to_new_folio(struct folio *dst, struct folio *src,
> -				enum migrate_mode mode)
> +		enum migrate_mode mode, bool already_copied)
>  {
>  	struct address_space *mapping = folio_mapping(src);
>  	int rc = -EAGAIN;
> @@ -1096,6 +1114,9 @@ static int move_to_new_folio(struct folio *dst, struct folio *src,
>  	VM_BUG_ON_FOLIO(!folio_test_locked(src), src);
>  	VM_BUG_ON_FOLIO(!folio_test_locked(dst), dst);
>  
> +	if (already_copied)
> +		dst->private = (void *)(unsigned long)PAGE_ALREADY_COPIED;
> +

IMHO, this appears to be an unusual way to pass arguments to a function.
Why not adjust the parameters of migrate_folio()?  How about turning enum
migrate_mode into a bitmask (migrate_flags)?

>  	if (!mapping)
>  		rc = migrate_folio(mapping, dst, src, mode);
>  	else if (mapping_inaccessible(mapping))
> @@ -1127,17 +1148,6 @@ static int move_to_new_folio(struct folio *dst, struct folio *src,
>  	return rc;
>  }
>  
> -/*
> - * 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_OLD_STATES = PAGE_WAS_MAPPED | PAGE_WAS_MLOCKED,
> -};
> -
>  static void __migrate_folio_record(struct folio *dst,
>  				   int old_page_state,
>  				   struct anon_vma *anon_vma)
> @@ -1353,7 +1363,7 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
>  static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private,
>  			      struct folio *src, struct folio *dst,
>  			      enum migrate_mode mode, enum migrate_reason reason,
> -			      struct list_head *ret)
> +			      struct list_head *ret, bool already_copied)
>  {
>  	int rc;
>  	int old_page_state = 0;
> @@ -1371,7 +1381,7 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private,
>  		goto out_unlock_both;
>  	}
>  
> -	rc = move_to_new_folio(dst, src, mode);
> +	rc = move_to_new_folio(dst, src, mode, already_copied);
>  	if (rc)
>  		goto out;
>  
> @@ -1519,7 +1529,7 @@ static int unmap_and_move_huge_page(new_folio_t get_new_folio,
>  	}
>  
>  	if (!folio_mapped(src))
> -		rc = move_to_new_folio(dst, src, mode);
> +		rc = move_to_new_folio(dst, src, mode, false);
>  
>  	if (page_was_mapped)
>  		remove_migration_ptes(src, !rc ? dst : src, ttu);
> @@ -1703,7 +1713,7 @@ static void migrate_folios_move(struct list_head *src_folios,
>  		struct list_head *ret_folios,
>  		struct migrate_pages_stats *stats,
>  		int *retry, int *thp_retry, int *nr_failed,
> -		int *nr_retry_pages)
> +		int *nr_retry_pages, bool already_copied)
>  {
>  	struct folio *folio, *folio2, *dst, *dst2;
>  	bool is_thp;
> @@ -1720,7 +1730,7 @@ static void migrate_folios_move(struct list_head *src_folios,
>  
>  		rc = migrate_folio_move(put_new_folio, private,
>  				folio, dst, mode,
> -				reason, ret_folios);
> +				reason, ret_folios, already_copied);
>  		/*
>  		 * The rules are:
>  		 *	0: folio will be freed
> @@ -1977,7 +1987,7 @@ static int migrate_pages_batch(struct list_head *from,
>  		migrate_folios_move(&unmap_folios, &dst_folios,
>  				put_new_folio, private, mode, reason,
>  				ret_folios, stats, &retry, &thp_retry,
> -				&nr_failed, &nr_retry_pages);
> +				&nr_failed, &nr_retry_pages, false);
>  	}
>  	nr_failed += retry;
>  	stats->nr_thp_failed += thp_retry;

---
Best Regards,
Huang, Ying