Re: [RFC V3 1/9] mm/migrate: factor out code in move_to_new_folio() and migrate_folio_move()

[Jonathan Cameron <jonathan.cameron@huawei.com>]

On Tue, 23 Sep 2025 17:47:36 +0000
Shivank Garg <shivankg@amd.com> wrote:

> From: Zi Yan <ziy@nvidia.com>
> 
> No function change is intended. The factored out code will be reused in
> an upcoming batched folio move function.
> 
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> Signed-off-by: Shivank Garg <shivankg@amd.com>
Hi.  A few code structure things inline.

The naming of the various helpers needs some more thought I think as
with it like this the loss of readability of existing code is painful.

Jonathan

> ---
>  mm/migrate.c | 106 ++++++++++++++++++++++++++++++++-------------------
>  1 file changed, 67 insertions(+), 39 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 9e5ef39ce73a..ad03e7257847 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1061,19 +1061,7 @@ static int fallback_migrate_folio(struct address_space *mapping,
>  	return migrate_folio(mapping, dst, src, mode);
>  }
>  
> -/*
> - * Move a src folio to a newly allocated dst folio.
> - *
> - * The src and dst folios are locked and the src folios was unmapped from
> - * the page tables.
> - *
> - * On success, the src folio was replaced by the dst folio.
> - *
> - * Return value:
> - *   < 0 - error code
> - *  MIGRATEPAGE_SUCCESS - success
> - */
> -static int move_to_new_folio(struct folio *dst, struct folio *src,
> +static int _move_to_new_folio_prep(struct folio *dst, struct folio *src,

I'm not sure the _ prefix is needed. Or maybe it should be __ like
__buffer_migrate_folio()


>  				enum migrate_mode mode)
>  {
>  	struct address_space *mapping = folio_mapping(src);
> @@ -1098,7 +1086,12 @@ static int move_to_new_folio(struct folio *dst, struct folio *src,
>  							mode);
>  	else
>  		rc = fallback_migrate_folio(mapping, dst, src, mode);
> +	return rc;

May be worth switching this whole function to early returns given we no longer
have a shared block of stuff to do at the end.

	if (!mapping)
		return migrate_folio(mapping, st, src, mode);

	if (mapping_inaccessible(mapping))
		return -EOPNOTSUPP;

	if (mapping->a_ops->migrate_folio)
		return mapping->a_ops->migrate_folio(mapping, dst, src, mode);

	return fallback_migrate_folio(mapping, dst, src, mode);

> +}
>  
> +static void _move_to_new_folio_finalize(struct folio *dst, struct folio *src,
> +				int rc)
> +{
>  	if (rc == MIGRATEPAGE_SUCCESS) {

Perhaps
	if (rc != MIGRATE_PAGE_SUCCESS)
		return rc;

	/*
	 * For pagecache folios,....

...

	return rc;

Unless other stuff is likely to get added in here.
Or drag the condition to the caller.

>  		/*
>  		 * For pagecache folios, src->mapping must be cleared before src
> @@ -1110,6 +1103,29 @@ static int move_to_new_folio(struct folio *dst, struct folio *src,
>  		if (likely(!folio_is_zone_device(dst)))
>  			flush_dcache_folio(dst);
>  	}
> +}
> +
> +/*
> + * Move a src folio to a newly allocated dst folio.
> + *
> + * The src and dst folios are locked and the src folios was unmapped from
> + * the page tables.
> + *
> + * On success, the src folio was replaced by the dst folio.
> + *
> + * Return value:
> + *   < 0 - error code
> + *  MIGRATEPAGE_SUCCESS - success
> + */
> +static int move_to_new_folio(struct folio *dst, struct folio *src,
> +			enum migrate_mode mode)
> +{
> +	int rc;
> +
> +	rc = _move_to_new_folio_prep(dst, src, mode);
> +
> +	_move_to_new_folio_finalize(dst, src, rc);
> +
>  	return rc;
>  }
>  
> @@ -1345,32 +1361,9 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
>  	return rc;
>  }
>  
> -/* Migrate the folio to the newly allocated folio in dst. */
> -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)
> +static void _migrate_folio_move_finalize1(struct folio *src, struct folio *dst,
> +					  int old_page_state)
>  {
> -	int rc;
> -	int old_page_state = 0;
> -	struct anon_vma *anon_vma = NULL;
> -	struct list_head *prev;
> -
> -	__migrate_folio_extract(dst, &old_page_state, &anon_vma);
> -	prev = dst->lru.prev;
> -	list_del(&dst->lru);
> -
> -	if (unlikely(page_has_movable_ops(&src->page))) {
> -		rc = migrate_movable_ops_page(&dst->page, &src->page, mode);
> -		if (rc)
> -			goto out;
> -		goto out_unlock_both;
> -	}
> -
> -	rc = move_to_new_folio(dst, src, mode);
> -	if (rc)
> -		goto out;
> -
>  	/*
>  	 * When successful, push dst to LRU immediately: so that if it
>  	 * turns out to be an mlocked page, remove_migration_ptes() will
> @@ -1386,8 +1379,12 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private,
>  
>  	if (old_page_state & PAGE_WAS_MAPPED)
>  		remove_migration_ptes(src, dst, 0);
> +}
>  
> -out_unlock_both:
> +static void _migrate_folio_move_finalize2(struct folio *src, struct folio *dst,
> +					  enum migrate_reason reason,
> +					  struct anon_vma *anon_vma)
> +{
>  	folio_unlock(dst);
>  	folio_set_owner_migrate_reason(dst, reason);
>  	/*
> @@ -1407,6 +1404,37 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private,
>  		put_anon_vma(anon_vma);
>  	folio_unlock(src);
>  	migrate_folio_done(src, reason);
> +}
> +
> +/* Migrate the folio to the newly allocated folio in dst. */
> +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)
> +{
> +	int rc;
> +	int old_page_state = 0;
> +	struct anon_vma *anon_vma = NULL;
> +	struct list_head *prev;
> +
> +	__migrate_folio_extract(dst, &old_page_state, &anon_vma);
> +	prev = dst->lru.prev;
> +	list_del(&dst->lru);
> +
> +	if (unlikely(page_has_movable_ops(&src->page))) {
> +		rc = migrate_movable_ops_page(&dst->page, &src->page, mode);
> +		if (rc)
> +			goto out;
> +		goto out_unlock_both;
I would drop this..
> +	}
and do
	} else {
		rc = move_to_new_folio(dst, src, mode);
		if (rc)
			goto out;
		_migrate_folio_move_finalize1(src, dst, old_page_state);
	}
	_migrate_folio_move_finalize2(src, dst, reason, anon_vma);
  
  	return rc;

This makes sense now as the amount of code indented more in this approach
is much smaller than it would have been before you factored stuff out.


> +
> +	rc = move_to_new_folio(dst, src, mode);
> +	if (rc)
> +		goto out;
> +
Hmm. These two functions might be useful but this is hurting readability
here.  Can we come up with some more meaningful names perhaps?

> +	_migrate_folio_move_finalize1(src, dst, old_page_state);
> +out_unlock_both:
> +	_migrate_folio_move_finalize2(src, dst, reason, anon_vma);
>  
>  	return rc;
>  out: