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

[David Hildenbrand]

On 02.07.25 12:34, Harry Yoo 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 at nvidia.com>
>> Signed-off-by: David Hildenbrand <david at redhat.com>
>> ---
> 
>> +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.
>> +	 */
>> +#ifdef CONFIG_BALLOON_COMPACTION
>> +	if (PageOffline(page))
>> +		/* Only balloon compaction sets PageOffline pages movable. */
>> +		return &balloon_mops;
>> +#endif /* CONFIG_BALLOON_COMPACTION */
>> +#if defined(CONFIG_ZSMALLOC) && defined(CONFIG_COMPACTION)
>> +	if (PageZsmalloc(page))
>> +		return &zsmalloc_mops;
>> +#endif /* defined(CONFIG_ZSMALLOC) && defined(CONFIG_COMPACTION) */
> 
> What happens if:
>    CONFIG_ZSMALLOC=y
>    CONFIG_TRANSPARENT_HUGEPAGE=n
>    CONFIG_COMPACTION=n
>    CONFIG_MIGRATION=y

Pages are never allocated from ZONE_MOVABLE/CMA and are not marked as 
having movable_ops, so we never end up in this function. See how 
zsmalloc.c deals with CONFIG_COMPACTION, especially how 
SetZsPageMovable() is a NOP without it.

As a side note, both should probably be moved from COMPACTION to 
MIGRATION. Although probably in practice, anybody enabling 
CONFIG_MIGRATION likely also enables CONFIG_COMPACTION.

-- 
Cheers,

David / dhildenb