Re: [PATCH v2 20/29] mm: convert "movable" flag in page->mapping to a page flag

[David Hildenbrand <david@redhat.com>]

On 04.07.25 12:25, David Hildenbrand wrote:
> Instead, let's use a page flag. As the page flag can result in
> false-positives, glue it to the page types for which we
> support/implement movable_ops page migration.
> 
> We are reusing PG_uptodate, that is for example used to track file
> system state and does not apply to movable_ops pages. So
> warning in case it is set in page_has_movable_ops() on other page types
> could result in false-positive warnings.
> 
> Likely we could set the bit using a non-atomic update: in contrast to
> page->mapping, we could have others trying to update the flags
> concurrently when trying to lock the folio. In
> isolate_movable_ops_page(), we already take care of that by checking if
> the page has movable_ops before locking it. Let's start with the atomic
> variant, we could later switch to the non-atomic variant once we are
> sure other cases are similarly fine. Once we perform the switch, we'll
> have to introduce __SETPAGEFLAG_NOOP().
> 
> Reviewed-by: Zi Yan <ziy@nvidia.com>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>   include/linux/balloon_compaction.h |  2 +-
>   include/linux/migrate.h            |  8 -----
>   include/linux/page-flags.h         | 54 ++++++++++++++++++++++++------
>   mm/compaction.c                    |  6 ----
>   mm/zpdesc.h                        |  2 +-
>   5 files changed, 46 insertions(+), 26 deletions(-)
> 
> diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
> index a8a1706cc56f3..b222b0737c466 100644
> --- a/include/linux/balloon_compaction.h
> +++ b/include/linux/balloon_compaction.h
> @@ -92,7 +92,7 @@ static inline void balloon_page_insert(struct balloon_dev_info *balloon,
>   				       struct page *page)
>   {
>   	__SetPageOffline(page);
> -	__SetPageMovable(page);
> +	SetPageMovableOps(page);
>   	set_page_private(page, (unsigned long)balloon);
>   	list_add(&page->lru, &balloon->pages);
>   }
> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> index 6aece3f3c8be8..acadd41e0b5cf 100644
> --- a/include/linux/migrate.h
> +++ b/include/linux/migrate.h
> @@ -103,14 +103,6 @@ static inline int migrate_huge_page_move_mapping(struct address_space *mapping,
>   
>   #endif /* CONFIG_MIGRATION */
>   
> -#ifdef CONFIG_COMPACTION
> -void __SetPageMovable(struct page *page);
> -#else
> -static inline void __SetPageMovable(struct page *page)
> -{
> -}
> -#endif
> -
>   #ifdef CONFIG_NUMA_BALANCING
>   int migrate_misplaced_folio_prepare(struct folio *folio,
>   		struct vm_area_struct *vma, int node);
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 4c27ebb689e3c..5f2b570735852 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -170,6 +170,11 @@ enum pageflags {
>   	/* non-lru isolated movable page */
>   	PG_isolated = PG_reclaim,
>   
> +#ifdef CONFIG_MIGRATION
> +	/* this is a movable_ops page (for selected typed pages only) */
> +	PG_movable_ops = PG_uptodate,
> +#endif
> +
>   	/* Only valid for buddy pages. Used to track pages that are reported */
>   	PG_reported = PG_uptodate,
>   
> @@ -698,9 +703,6 @@ PAGEFLAG_FALSE(VmemmapSelfHosted, vmemmap_self_hosted)
>    * bit; and then folio->mapping points, not to an anon_vma, but to a private
>    * structure which KSM associates with that merged page.  See ksm.h.
>    *
> - * PAGE_MAPPING_KSM without PAGE_MAPPING_ANON is used for non-lru movable
> - * page and then folio->mapping points to a struct movable_operations.
> - *
>    * Please note that, confusingly, "folio_mapping" refers to the inode
>    * address_space which maps the folio from disk; whereas "folio_mapped"
>    * refers to user virtual address space into which the folio is mapped.
> @@ -743,13 +745,6 @@ static __always_inline bool PageAnon(const struct page *page)
>   {
>   	return folio_test_anon(page_folio(page));
>   }
> -
> -static __always_inline bool page_has_movable_ops(const struct page *page)
> -{
> -	return ((unsigned long)page->mapping & PAGE_MAPPING_FLAGS) ==
> -				PAGE_MAPPING_MOVABLE;
> -}
> -
>   #ifdef CONFIG_KSM
>   /*
>    * A KSM page is one of those write-protected "shared pages" or "merged pages"
> @@ -1133,6 +1128,45 @@ bool is_free_buddy_page(const struct page *page);
>   
>   PAGEFLAG(Isolated, isolated, PF_ANY);
>   
> +#ifdef CONFIG_MIGRATION
> +/*
> + * This page is migratable through movable_ops (for selected typed pages
> + * only).
> + *
> + * Page migration of such pages might fail, for example, if the page is
> + * already isolated by somebody else, or if the page is about to get freed.
> + *
> + * While a subsystem might set selected typed pages that support page migration
> + * as being movable through movable_ops, it must never clear this flag.
> + *
> + * This flag is only cleared when the page is freed back to the buddy.
> + *
> + * Only selected page types support this flag (see page_movable_ops()) and
> + * the flag might be used in other context for other pages. Always use
> + * page_has_movable_ops() instead.
> + */
> +TESTPAGEFLAG(MovableOps, movable_ops, PF_NO_TAIL);
> +SETPAGEFLAG(MovableOps, movable_ops, PF_NO_TAIL);
> +#else /* !CONFIG_MIGRATION */
> +TESTPAGEFLAG_FALSE(MovableOps, movable_ops);
> +SETPAGEFLAG_NOOP(MovableOps, movable_ops);
> +#endif /* CONFIG_MIGRATION */
> +
> +/**
> + * page_has_movable_ops - test for a movable_ops page
> + * @page The page to test.
> + *
> + * Test whether this is a movable_ops page. Such pages will stay that
> + * way until freed.
> + *
> + * Returns true if this is a movable_ops page, otherwise false.
> + */
> +static inline bool page_has_movable_ops(const struct page *page)
> +{
> +	return PageMovableOps(page) &&
> +	       (PageOffline(page) || PageZsmalloc(page));
> +}
> +

The following fixup on top:

 From 3a52911a299d3328d9fa2aeba00170240795702d Mon Sep 17 00:00:00 2001
From: David Hildenbrand <david@redhat.com>
Date: Fri, 11 Jul 2025 11:57:43 +0200
Subject: [PATCH] fixup: "mm: convert "movable" flag in page->mapping to a page
  flag"

We're missing a ":".

Signed-off-by: David Hildenbrand <david@redhat.com>
---
  include/linux/page-flags.h | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 970600d79daca..8e4d6eda8a8d6 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -1150,7 +1150,7 @@ PAGEFLAG_FALSE(MovableOpsIsolated, movable_ops_isolated);
  
  /**
   * page_has_movable_ops - test for a movable_ops page
- * @page The page to test.
+ * @page: The page to test.
   *
   * Test whether this is a movable_ops page. Such pages will stay that
   * way until freed.
-- 
2.50.1


-- 
Cheers,

David / dhildenb