On 01.07.25 14:12, Lorenzo Stoakes wrote:

On Mon, Jun 30, 2025 at 03:00:00PM +0200, David Hildenbrand wrote:

quoted

... instead, look them up statically based on the page type. Maybe in the
future we want a registration interface? At least for now, it can be
easily handled using the two page types that actually support page
migration.

The remaining usage of page->mapping is to flag such pages as actually
being movable (having movable_ops), which we will change next.

Reviewed-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: David Hildenbrand <redacted>

See comment below, this feels iffy in the long run but ok as an interim measure.

So:

Reviewed-by: Lorenzo Stoakes <redacted>

quoted

---
  include/linux/balloon_compaction.h |  2 +-
  include/linux/migrate.h            | 14 ++------------
  include/linux/zsmalloc.h           |  2 ++
  mm/balloon_compaction.c            |  1 -
  mm/compaction.c                    |  5 ++---
  mm/migrate.c                       | 23 +++++++++++++++++++++++
  mm/zpdesc.h                        |  5 ++---
  mm/zsmalloc.c                      |  8 +++-----
  8 files changed, 35 insertions(+), 25 deletions(-)

diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
index 9bce8e9f5018c..a8a1706cc56f3 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, &balloon_mops);
+	__SetPageMovable(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 e04035f70e36f..6aece3f3c8be8 100644
--- a/include/linux/migrate.h
+++ b/include/linux/migrate.h

@@ -104,23 +104,13 @@ static inline int migrate_huge_page_move_mapping(struct address_space *mapping,
  #endif /* CONFIG_MIGRATION */

  #ifdef CONFIG_COMPACTION
-void __SetPageMovable(struct page *page, const struct movable_operations *ops);
+void __SetPageMovable(struct page *page);
  #else
-static inline void __SetPageMovable(struct page *page,
-		const struct movable_operations *ops)
+static inline void __SetPageMovable(struct page *page)
  {
  }
  #endif

-static inline
-const struct movable_operations *page_movable_ops(struct page *page)
-{
-	VM_WARN_ON_ONCE_PAGE(!page_has_movable_ops(page), page);
-
-	return (const struct movable_operations *)
-		((unsigned long)page->mapping - PAGE_MAPPING_MOVABLE);
-}
-
  #ifdef CONFIG_NUMA_BALANCING
  int migrate_misplaced_folio_prepare(struct folio *folio,
  		struct vm_area_struct *vma, int node);

diff --git a/include/linux/zsmalloc.h b/include/linux/zsmalloc.h
index 13e9cc5490f71..f3ccff2d966cd 100644
--- a/include/linux/zsmalloc.h
+++ b/include/linux/zsmalloc.h

@@ -46,4 +46,6 @@ void zs_obj_read_end(struct zs_pool *pool, unsigned long handle,
  void zs_obj_write(struct zs_pool *pool, unsigned long handle,
  		  void *handle_mem, size_t mem_len);

+extern const struct movable_operations zsmalloc_mops;
+
  #endif

diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
index e4f1a122d786b..2a4a649805c11 100644
--- a/mm/balloon_compaction.c
+++ b/mm/balloon_compaction.c

@@ -253,6 +253,5 @@ const struct movable_operations balloon_mops = {
  	.isolate_page = balloon_page_isolate,
  	.putback_page = balloon_page_putback,
  };
-EXPORT_SYMBOL_GPL(balloon_mops);

  #endif /* CONFIG_BALLOON_COMPACTION */

diff --git a/mm/compaction.c b/mm/compaction.c
index 41fd6a1fe9a33..348eb754cb227 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c

@@ -114,11 +114,10 @@ static unsigned long release_free_list(struct list_head *freepages)
  }

  #ifdef CONFIG_COMPACTION
-void __SetPageMovable(struct page *page, const struct movable_operations *mops)
+void __SetPageMovable(struct page *page)
  {
  	VM_BUG_ON_PAGE(!PageLocked(page), page);
-	VM_BUG_ON_PAGE((unsigned long)mops & PAGE_MAPPING_MOVABLE, page);
-	page->mapping = (void *)((unsigned long)mops | PAGE_MAPPING_MOVABLE);
+	page->mapping = (void *)(PAGE_MAPPING_MOVABLE);
  }
  EXPORT_SYMBOL(__SetPageMovable);

diff --git a/mm/migrate.c b/mm/migrate.c
index 15d3c1031530c..c6c9998014ec8 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c

@@ -43,6 +43,8 @@
  #include <linux/sched/sysctl.h>
  #include <linux/memory-tiers.h>
  #include <linux/pagewalk.h>
+#include <linux/balloon_compaction.h>
+#include <linux/zsmalloc.h>

  #include <asm/tlbflush.h>

@@ -51,6 +53,27 @@
  #include "internal.h"
  #include "swap.h"

+static const struct movable_operations *page_movable_ops(struct page *page)
+{
+	VM_WARN_ON_ONCE_PAGE(!page_has_movable_ops(page), page);
+
+	/*
+	 * If we enable page migration for a page of a certain type by marking
+	 * it as movable, the page type must be sticky until the page gets freed
+	 * back to the buddy.
+	 */

Ah now this makes more sense...

quoted

+#ifdef CONFIG_BALLOON_COMPACTION
+	if (PageOffline(page))
+		/* Only balloon compaction sets PageOffline pages movable. */
+		return &balloon_mops;

So it's certain that if we try to invoke movable ops, and it's the balloon
compaction case, the page will be offline?

Yes. The page must be marked as having movable_ops by the user. The next 
patch reworks that as well.

A PageOffline page without movable_ops will never end up here 
(page_has_movable_ops() == false).

quoted

+#endif /* CONFIG_BALLOON_COMPACTION */
+#if defined(CONFIG_ZSMALLOC) && defined(CONFIG_COMPACTION)
+	if (PageZsmalloc(page))

And same question only for ZS malloc.

Same thing.

quoted

+		return &zsmalloc_mops;
+#endif /* defined(CONFIG_ZSMALLOC) && defined(CONFIG_COMPACTION) */
+	return NULL;
+}

This is kind of sketchy as it's baking in assumptions implicitly, so I hope we
can find an improved way of doing this later, even if it's about providing
e.g. is_ballon_movable_ops_page() and is_zsmalloc_movable_ops_page() predicates
that abstract this code + placing them in the relevant code so it's at least
obvious to people working on this stuff that this needs to be considered.

But ok as a means of getting away from having to have the hook object encoded.

Yeah, not sure yet how to clean that up in the future. As I stated 
somewhere, maybe we just want a registration interface to handle a 
specific page type. But for handling the two known in-tree users, this 
should get us going.

Thanks!

-- 
Cheers,

David / dhildenb