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

[Miaohe Lin]

On 2022/6/2 16:47, David Hildenbrand wrote:
> On 02.06.22 09:40, Miaohe Lin wrote:
>> On 2022/6/1 18:31, David Hildenbrand wrote:
>>> On 31.05.22 14:37, Miaohe Lin wrote:
>>>> On 2022/5/31 19:59, David Hildenbrand wrote:
>>>>> Sorry for the late reply, was on vacation.
>>>>
>>>> That's all right. Hope you have a great time. ;)
>>>>
>>>>>
>>>>>>>>
>>>>>>>> But for isolated page, PageLRU is cleared. So when the isolated page is released, __clear_page_lru_flags
>>>>>>>> won't be called. So we have to clear the PG_active and PG_unevictable here manully. So I think
>>>>>>>> this code block works. Or am I miss something again?
>>>>>>>
>>>>>>> Let's assume the following: page as freed by the owner and we enter
>>>>>>> unmap_and_move().
>>>>>>>
>>>>>>>
>>>>>>> #1: enter unmap_and_move() // page_count is 1
>>>>>>> #2: enter isolate_movable_page() // page_count is 1
>>>>>>> #2: get_page_unless_zero() // page_count is now 2
>>>>>>> #1: if (page_count(page) == 1) { // does not trigger
>>>>>>> #2: put_page(page); // page_count is now 1
>>>>>>> #1: put_page(page); // page_count is now 0 -> freed
>>>>>>>
>>>>>>>
>>>>>>> #1 will trigger __put_page() -> __put_single_page() ->
>>>>>>> __page_cache_release() will not clear the flags because it's not an LRU
>>>>>>> page at that point in time, right (-> isolated)?
>>>>>>
>>>>>> Sorry, you're right. I thought the old page will be freed via putback_lru_page which will
>>>>>> set PageLRU back instead of put_page directly. So if the above race occurs, PG_active and
>>>>>> PG_unevictable will remain set while page goes to the buddy and check_free_page will complain
>>>>>> about it. But it seems this is never witnessed?
>>>>>
>>>>> Maybe
>>>>>
>>>>> a) we were lucky so far and didn't trigger it
>>>>> b) the whole code block is dead code because we are missing something
>>>>> c) we are missing something else :)
>>>>
>>>> I think I found the things we missed in another email [1].
>>>> [1]: https://lore.kernel.org/all/948ea45e-3b2b-e16c-5b8c-4c34de0ea593@xxxxxxxxxx/
>>>>
>>>> Paste the main content of [1] here:
>>>>
>>>> "
>>>> There are 3 cases in unmap_and_move:
>>>>
>>>> 1.page is freed through "if (page_count(page) == 1)" code block. This works
>>>> as PG_active and PG_unevictable are cleared here.
>>>>
>>>> 2. Failed to migrate the page. The page won't be release so we don't care about it.
>>>
>>> Right, page is un-isolated.
>>>
>>>>
>>>> 3. The page is migrated successfully. The PG_active and PG_unevictable are cleared
>>>> via folio_migrate_flags():
>>>>
>>>> 	if (folio_test_clear_active(folio)) {
>>>> 		VM_BUG_ON_FOLIO(folio_test_unevictable(folio), folio);
>>>> 		folio_set_active(newfolio);
>>>> 	} else if (folio_test_clear_unevictable(folio))
>>>> 		folio_set_unevictable(newfolio);
>>>
>>> Right.
>>>
>>>>
>>>> For the above race case, the page won't be freed through "if (page_count(page) == 1)" code block.
>>>> It will just be migrated and freed via put_page() after folio_migrate_flags() having cleared PG_active
>>>> and PG_unevictable.
>>>> "
>>>> Or Am I miss something again? :)
>>>
>>> For #1, I'm still not sure what would happen on a speculative reference.
>>>
>>> It's worth summarizing that
>>>
>>> a) free_pages_prepare() will clear both flags via page->flags &=
>>> ~PAGE_FLAGS_CHECK_AT_PREP;
>>>
>>> b) free_pages_prepare() will bail out if any flag is set in
>>> check_free_page().
>>>
>>> As we've never seen b) in the wild, this certainly has low priority, and
>>> maybe it really cannot happen right now.
>>>
>>> However, maybe really allowing these flags to be set when freeing the
>>> page and removing the "page_count(page) == 1" case from migration code
>>> would be the clean thing to do.
>>
>> IMHO, check_free_page is used to catch possible problem. There's the comment of PAGE_FLAGS_CHECK_AT_FREE:
>>
>> /*
>>  * Flags checked when a page is freed.  Pages being freed should not have
>>  * these flags set.  If they are, there is a problem.
>>  */
>> #define PAGE_FLAGS_CHECK_AT_FREE
>>
>> There might be an assumption: when page is freed, it shouldn't be an active or unevictable page. It should be
>> inactive and evictable. So allowing these flags to be set when freeing the page might not be a good idea?
> 
> Yeah, and we'd be lifting that restriction because there is good reason
> to do so.
> 
> Maybe we *could* special case for isolated pages; however, that adds
> runtime overhead. Of course, we could perform different checks for e.g.,
> DEBUG_VM vs !DEBUG_VM.

I found there is one assumption about PG_active and PG_unevictable, i.e. in __folio_clear_lru_flags:

	/* this shouldn't happen, so leave the flags to bad_page() */
	if (folio_test_active(folio) && folio_test_unevictable(folio))
		return;

If PG_active and PG_unevictable are both set, this case will be caught in the bad_page() via check_free_page().
There might be some other assumptions about PG_active and PG_unevictable. So I think it's not safe to lift that
restriction.

But maybe we could limit this check within DEBUG_VM as you suggested. Am I supposed to do it?

Thanks!

>