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

David Hildenbrand
2022-06-09T12:50:49Z

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