Re: [PATCH v1 19/29] mm: stop storing migration_ops in page->mapping

David Hildenbrand
2025-07-01T12:41:11Z

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@nvidia.com>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>
> See comment below, this feels iffy in the long run but ok as an interim measure.
>
> So:
>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>
>> ---
>> 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