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

David Hildenbrand
2025-07-03T07:45:36Z

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