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

[David Hildenbrand]

On 09.05.22 10:51, Miaohe Lin wrote:
> 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?

At least the code we have now:

if (unlikely(__PageMovable(page)))
	ClearPageIsolated(page);

Should be dead code. So PG_isolated could remain set.

If PG_isolated is still set, it will get cleared in the buddy when
freeing the page via

	page->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;

> 
>>
>>
>> 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.

How can you prevent any kind of speculative references?

See isolate_movable_page() as an example, which grabs a speculative
reference to then find out that the page is already isolated by someone
else, to then back off.

-- 
Thanks,

David / dhildenb