Re: [PATCH v2 03/19] fs: Add aops->migrate_folio

[David Hildenbrand <david@redhat.com>]

On 08.06.22 17:02, Matthew Wilcox (Oracle) wrote:
> Provide a folio-based replacement for aops->migratepage.  Update the
> documentation to document migrate_folio instead of migratepage.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> ---
>  Documentation/filesystems/locking.rst |  5 ++--
>  Documentation/filesystems/vfs.rst     | 13 ++++++-----
>  Documentation/vm/page_migration.rst   | 33 ++++++++++++++-------------
>  include/linux/fs.h                    |  4 +++-
>  mm/compaction.c                       |  4 +++-
>  mm/migrate.c                          | 11 +++++----
>  6 files changed, 40 insertions(+), 30 deletions(-)
> 
> diff --git a/Documentation/filesystems/locking.rst b/Documentation/filesystems/locking.rst
> index c0fe711f14d3..3d28b23676bd 100644
> --- a/Documentation/filesystems/locking.rst
> +++ b/Documentation/filesystems/locking.rst
> @@ -253,7 +253,8 @@ prototypes::
>  	void (*free_folio)(struct folio *);
>  	int (*direct_IO)(struct kiocb *, struct iov_iter *iter);
>  	bool (*isolate_page) (struct page *, isolate_mode_t);
> -	int (*migratepage)(struct address_space *, struct page *, struct page *);
> +	int (*migrate_folio)(struct address_space *, struct folio *dst,
> +			struct folio *src, enum migrate_mode);
>  	void (*putback_page) (struct page *);

isolate_page/putback_page are leftovers from the previous patch, no?

>  	int (*launder_folio)(struct folio *);
>  	bool (*is_partially_uptodate)(struct folio *, size_t from, size_t count);
> @@ -281,7 +282,7 @@ release_folio:		yes
>  free_folio:		yes
>  direct_IO:
>  isolate_page:		yes
> -migratepage:		yes (both)
> +migrate_folio:		yes (both)
>  putback_page:		yes

Dito.

>  launder_folio:		yes
>  is_partially_uptodate:	yes
> diff --git a/Documentation/filesystems/vfs.rst b/Documentation/filesystems/vfs.rst
> index a08c652467d7..3ae1b039b03f 100644
> --- a/Documentation/filesystems/vfs.rst
> +++ b/Documentation/filesystems/vfs.rst
> @@ -740,7 +740,8 @@ cache in your filesystem.  The following members are defined:
>  		/* isolate a page for migration */
>  		bool (*isolate_page) (struct page *, isolate_mode_t);
>  		/* migrate the contents of a page to the specified target */
> -		int (*migratepage) (struct page *, struct page *);
> +		int (*migrate_folio)(struct mapping *, struct folio *dst,
> +				struct folio *src, enum migrate_mode);
>  		/* put migration-failed page back to right list */
>  		void (*putback_page) (struct page *);

Dito.

>  		int (*launder_folio) (struct folio *);
> @@ -935,12 +936,12 @@ cache in your filesystem.  The following members are defined:
>  	is successfully isolated, VM marks the page as PG_isolated via
>  	__SetPageIsolated.
>  
> -``migrate_page``
> +``migrate_folio``
>  	This is used to compact the physical memory usage.  If the VM
> -	wants to relocate a page (maybe off a memory card that is
> -	signalling imminent failure) it will pass a new page and an old
> -	page to this function.  migrate_page should transfer any private
> -	data across and update any references that it has to the page.
> +	wants to relocate a folio (maybe from a memory device that is
> +	signalling imminent failure) it will pass a new folio and an old
> +	folio to this function.  migrate_folio should transfer any private
> +	data across and update any references that it has to the folio.
>  
>  ``putback_page``
>  	Called by the VM when isolated page's migration fails.

Dito.

> diff --git a/Documentation/vm/page_migration.rst b/Documentation/vm/page_migration.rst
> index 8c5cb8147e55..e0f73ddfabb1 100644
> --- a/Documentation/vm/page_migration.rst
> +++ b/Documentation/vm/page_migration.rst
> @@ -181,22 +181,23 @@ which are function pointers of struct address_space_operations.
>     Once page is successfully isolated, VM uses page.lru fields so driver
>     shouldn't expect to preserve values in those fields.
>  
> -2. ``int (*migratepage) (struct address_space *mapping,``
> -|	``struct page *newpage, struct page *oldpage, enum migrate_mode);``
> -
> -   After isolation, VM calls migratepage() of driver with the isolated page.
> -   The function of migratepage() is to move the contents of the old page to the
> -   new page
> -   and set up fields of struct page newpage. Keep in mind that you should
> -   indicate to the VM the oldpage is no longer movable via __ClearPageMovable()
> -   under page_lock if you migrated the oldpage successfully and returned
> -   MIGRATEPAGE_SUCCESS. If driver cannot migrate the page at the moment, driver
> -   can return -EAGAIN. On -EAGAIN, VM will retry page migration in a short time
> -   because VM interprets -EAGAIN as "temporary migration failure". On returning
> -   any error except -EAGAIN, VM will give up the page migration without
> -   retrying.
> -
> -   Driver shouldn't touch the page.lru field while in the migratepage() function.
> +2. ``int (*migrate_folio) (struct address_space *mapping,``
> +|	``struct folio *dst, struct folio *src, enum migrate_mode);``
> +
> +   After isolation, VM calls the driver's migrate_folio() with the
> +   isolated folio.  The purpose of migrate_folio() is to move the contents
> +   of the source folio to the destination folio and set up the fields
> +   of destination folio.  Keep in mind that you should indicate to the
> +   VM the source folio is no longer movable via __ClearPageMovable()
> +   under folio if you migrated the source successfully and returned
> +   MIGRATEPAGE_SUCCESS.  If driver cannot migrate the folio at the
> +   moment, driver can return -EAGAIN. On -EAGAIN, VM will retry folio
> +   migration in a short time because VM interprets -EAGAIN as "temporary
> +   migration failure".  On returning any error except -EAGAIN, VM will
> +   give up the folio migration without retrying.
> +
> +   Driver shouldn't touch the folio.lru field while in the migrate_folio()
> +   function.
>  
>  3. ``void (*putback_page)(struct page *);``

Hmm, here it's a bit more complicated now, because we essentially have
two paths: LRU+migrate_folio or !LRU+movable_ops
(isolate/migrate/putback page)



-- 
Thanks,

David / dhildenb