Re: [PATCH v1 19/29] mm: stop storing migration_ops in page->mapping
From: David Hildenbrand
Date: Tue Jul 01 2025 - 08:42:01 EST
On 01.07.25 14:12, Lorenzo Stoakes wrote:
On Mon, Jun 30, 2025 at 03:00:00PM +0200, David Hildenbrand wrote:
... 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@xxxxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
See comment below, this feels iffy in the long run but ok as an interim measure.
So:
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
---
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...
+#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).
+#endif /* CONFIG_BALLOON_COMPACTION */
+#if defined(CONFIG_ZSMALLOC) && defined(CONFIG_COMPACTION)
+ if (PageZsmalloc(page))
And same question only for ZS malloc.
Same thing.
+ 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