Re: [PATCH RFC 27/29] docs/mm: convert from "Non-LRU page migration" to "movable_ops page migration"

[Zi Yan]

On 18 Jun 2025, at 13:40, David Hildenbrand wrote:

> Let's bring the docs up-to-date.
>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>  Documentation/mm/page_migration.rst | 39 ++++++++++++++++++++---------
>  1 file changed, 27 insertions(+), 12 deletions(-)
>
> diff --git a/Documentation/mm/page_migration.rst b/Documentation/mm/page_migration.rst
> index 519b35a4caf5b..a448e95e0a98e 100644
> --- a/Documentation/mm/page_migration.rst
> +++ b/Documentation/mm/page_migration.rst
> @@ -146,18 +146,33 @@ Steps:
>  18. The new page is moved to the LRU and can be scanned by the swapper,
>      etc. again.
>
> -Non-LRU page migration
> -======================
> -
> -Although migration originally aimed for reducing the latency of memory
> -accesses for NUMA, compaction also uses migration to create high-order
> -pages.  For compaction purposes, it is also useful to be able to move
> -non-LRU pages, such as zsmalloc and virtio-balloon pages.
> -
> -If a driver wants to make its pages movable, it should define a struct
> -movable_operations.  It then needs to call __SetPageMovable() on each
> -page that it may be able to move.  This uses the ``page->mapping`` field,
> -so this field is not available for the driver to use for other purposes.
> +movable_ops page migration
> +==========================
> +
> +Selected typed, non-folio pages (e.g., pages inflated in a memory balloon,

This is a great clarification. Thanks.

> +zsmalloc pages) can be migrated using the movable_ops migration framework.
> +
> +The "struct movable_operations" provide callbacks specific to a page type
> +for isolating, migrating and un-isolating (putback) these pages.
> +
> +Once a page is indicated as having movable_ops, that condition must not
> +change until the page was freed back to the buddy. This includes not
> +changing/clearing the page type and not changing/clearing the
> +PG_movable_ops page flag.
> +
> +Arbitrary drivers cannot currently make use of this framework, as it
> +requires:
> +
> +(a) a page type
> +(b) indicating them as possibly having movable_ops in page_has_movable_ops()
> +    based on the page type
> +(c) returning the movable_ops from page_has_movable_ops() based on the page
> +    type
> +(d) not reusing the PG_movable_ops and PG_movable_ops_isolated page flags
> +    for other purposes
> +
> +For example, balloon drivers can make use of this framework through the
> +balloon-compaction framework residing in the core kernel.
>
>  Monitoring Migration
>  =====================

Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>

--
Best Regards,
Yan, Zi