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

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Message-ID: <6a83e3e1-ab1d-409b-8395-df363321cade@redhat.com>
Date: Thu, 3 Jul 2025 09:45:29 +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 03.07.25 05:22, Sergey Senozhatsky wrote:
> On (25/07/03 11:28), Sergey Senozhatsky 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
>>
>> Oh, that makes two of us.
> 
> I sort of wonder if zs_page_migrate() VM_BUG_ON_PAGE() removal and
> zspage check addition need to be landed outside of this series, as
> a zsmalloc fixup.

Not sure if there is real value for that; given the review status, I 
assume this series won't take too long to be ready for upstream. Of 
course, if that is not the case we could try pulling them out.

-- 
Cheers,

David / dhildenb