Re: [PATCH v1 12/29] mm/zsmalloc: stop using __ClearPageMovable()

David Hildenbrand
2025-07-02T10:55:08Z

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