Re: [PATCH v2 2/4] mm/migration: remove unneeded lock page and PageMovable check

[Miaohe Lin]

On 2022/4/29 18:07, David Hildenbrand wrote:
> On 25.04.22 15:27, Miaohe Lin wrote:
>> When non-lru movable page was freed from under us, __ClearPageMovable must
>> have been done. Even if it's not done, ClearPageIsolated here won't hurt
>> as page will be freed anyway. So we can thus remove unneeded lock page and
>> PageMovable check here.
>>
>> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
>> Reviewed-by: Christoph Hellwig <hch@xxxxxx>
>> ---
>>  mm/migrate.c | 8 ++------
>>  1 file changed, 2 insertions(+), 6 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index b779646665fe..0fc4651b3e39 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1093,12 +1093,8 @@ static int unmap_and_move(new_page_t get_new_page,
>>  		/* page was freed from under us. So we are done. */
>>  		ClearPageActive(page);
>>  		ClearPageUnevictable(page);
>> -		if (unlikely(__PageMovable(page))) {
>> -			lock_page(page);
>> -			if (!PageMovable(page))
>> -				ClearPageIsolated(page);
>> -			unlock_page(page);
>> -		}
>> +		if (unlikely(__PageMovable(page)))
>> +			ClearPageIsolated(page);
>>  		goto out;
>>  	}
> 
> Hm, that code+change raises a couple of questions.
> 
> We're doing here the same as in putback_movable_pages(). So I guess the
> difference here is that the caller did release the reference while the
> page was isolated, while we don't assume the same in
> putback_movable_pages().

Agree.

> 
> 
> Shouldn't whoever owned the page have cleared that? IOW, is it even
> valid that we see a movable or isolated page here (WARN/BUG?)?
> 
> At least for balloon compaction, I remember that __PageMovable() is
> properly cleared before freeing it via balloon_page_delete().

z3fold, zsmalloc will do __ClearPageMovable when the page is going to be released.
So I think we shouldn't see a movable page here:

void __ClearPageMovable(struct page *page)
{
	VM_BUG_ON_PAGE(!PageMovable(page), page);
	/*
	 * Clear registered address_space val with keeping PAGE_MAPPING_MOVABLE
	 * flag so that VM can catch up released page by driver after isolation.
	 * With it, VM migration doesn't try to put it back.
	 */
	page->mapping = (void *)((unsigned long)page->mapping &
				PAGE_MAPPING_MOVABLE);
}

But it seems there is no guarantee for PageIsolated flag. Or am I miss something?

> 
> 
> Also, I am not sure how reliable that page count check is here: if we'd
> have another speculative reference to the page, we might see
> "page_count(page) > 1" and not take that path, although the previous
> owner released the last reference.

IIUC, there should not be such speculative reference. The driver should have taken care
of it.

Thanks!

> 
>