linux-kernel - Re: [PATCH v1 19/29] mm: stop storing migration

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Message-ID: <edb588a4-41c1-4108-9d86-fa69e1db5237@redhat.com>
Date: Wed, 2 Jul 2025 13:51:52 +0200
From: David Hildenbrand <david@...hat.com>
To: Harry Yoo <harry.yoo@...cle.com>
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Subject: Re: [PATCH v1 19/29] mm: stop storing migration_ops in page->mapping

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@...dia.com>
>>>> Signed-off-by: David Hildenbrand <david@...hat.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@...cle.com>

Thanks!

-- 
Cheers,

David / dhildenb