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

David Hildenbrand
2025-07-02T11:51:59Z

On 02.07.25 13:43, Harry Yoo wrote:
> On Wed, Jul 02, 2025 at 01:04:05PM +0200, David Hildenbrand wrote:
>> 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@nvidia.com>
>>>> Signed-off-by: David Hildenbrand <david@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
>
> I don't understand how that's true, neither zram nor zsmalloc clears
> __GFP_MOVABLE when CONFIG_COMPACTION=n?
>
> ...Or perhaps I'm still missing some pieces ;)

You might have found a bug in zsmalloc then :) Without support for compaction, we
must clear __GFP_MOVABLE in alloc_zpdesc() I assume.

Do you have the capacity to look into that and send a fix if really broken?

In balloon compaction code we properly handle that.

>
>> are not marked as having movable_ops, so we never end up in this function.
>
> Right.
>
>> See how zsmalloc.c deals with CONFIG_COMPACTION, especially how
>> SetZsPageMovable() is a NOP without it.
>
> Right.
>
> Now I see what I was missing in the previous reply.
> Thanks!
>
> Please feel free to add:
>
> Reviewed-by: Harry Yoo <harry.yoo@oracle.com>

Thanks!

--
Cheers,

David / dhildenb