linux-kernel - Re: [PATCH v1 12/29] mm/zsmalloc: stop using _

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Message-ID: <11de6ae0-d4ec-43d5-a82e-146d82f17fff@redhat.com>
Date: Wed, 2 Jul 2025 12:55:02 +0200
From: David Hildenbrand <david@...hat.com>
To: Sergey Senozhatsky <senozhatsky@...omium.org>
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Subject: Re: [PATCH v1 12/29] mm/zsmalloc: stop using __ClearPageMovable()

On 02.07.25 12:10, Sergey Senozhatsky wrote:
> On (25/07/02 10:25), David Hildenbrand wrote:
>> On 02.07.25 10:11, Sergey Senozhatsky wrote:
>>> On (25/06/30 14:59), David Hildenbrand wrote:
>>> [..]
>>>>    static int zs_page_migrate(struct page *newpage, struct page *page,
>>>> @@ -1736,6 +1736,13 @@ static int zs_page_migrate(struct page *newpage, struct page *page,
>>>>    	unsigned long old_obj, new_obj;
>>>>    	unsigned int obj_idx;
>>>> +	/*
>>>> +	 * TODO: nothing prevents a zspage from getting destroyed while
>>>> +	 * isolated: we should disallow that and defer it.
>>>> +	 */
>>>
>>> Can you elaborate?
>>
>> We can only free a zspage in free_zspage() while the page is locked.
>>
>> After we isolated a zspage page for migration (under page lock!), we drop
>                        ^^ a physical page? (IOW zspage chain page?)
> 
>> the lock again, to retake the lock when trying to migrate it.
>>
>> That means, there is a window where a zspage can be freed although the page
>> is isolated for migration.
> 
> I see, thanks.  Looks somewhat fragile.  Is this a new thing?

No, it's been like that forever. And I was surprised that only zsmalloc 
behaves that way -- balloon implements isolation as one would expect it 
(disallow freeing while isolated).

> 
>> While we currently keep that working (as far as I can see), in the future we
>> want to remove that support from the core.
> 
> Maybe comment can more explicitly distinguish zspage isolation and
> physical page (zspage chain) isolation?  zspages can get isolated
> for compaction (defragmentation), for instance, which is a different
> form of isolation.

Well, it's confusing, as we have MM compaction (-> migration) and 
apparently zs_compact.

I'll try to clarify that we are talking about isolation for page 
migration purposes.

-- 
Cheers,

David / dhildenb