linux-kernel - Re: [PATCH v1 27/29] docs/mm: convert from "Non-LRU page migration" to "movable

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Message-ID: <8545452f-5b0b-4a6b-a473-5a5fad79f390@lucifer.local>
Date: Tue, 1 Jul 2025 14:19:05 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-doc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
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        Shakeel Butt <shakeel.butt@...ux.dev>
Subject: Re: [PATCH v1 27/29] docs/mm: convert from "Non-LRU page migration"
 to "movable_ops page migration"

On Mon, Jun 30, 2025 at 03:00:08PM +0200, David Hildenbrand wrote:
> Let's bring the docs up-to-date.
>
> Reviewed-by: Zi Yan <ziy@...dia.com>
> Signed-off-by: David Hildenbrand <david@...hat.com>

LGTM, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

> ---
>  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..d611bc21920d7 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
> +==========================

Bye bye inaccurate reference to LRU :)

> +
> +Selected typed, non-folio pages (e.g., pages inflated in a memory balloon,
> +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 infrastructure residing in the core kernel.
>
>  Monitoring Migration
>  =====================
> --
> 2.49.0
>