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

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Message-ID: <7ca676a9-1f51-47f7-0245-d041d075a440@huawei.com>
Date:   Tue, 31 May 2022 20:37:51 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     David Hildenbrand <david@...hat.com>
CC:     <ying.huang@...el.com>, <hch@....de>, <dhowells@...hat.com>,
        <cl@...ux.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, <akpm@...ux-foundation.org>,
        <mike.kravetz@...cle.com>, <naoya.horiguchi@....com>,
        Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH v2 2/4] mm/migration: remove unneeded lock page and
 PageMovable check

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@huawei.com/

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.

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);

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? :)

> 
>>
>>>
>>> We did not run that code block that would clear PG_active and
>>> PG_unevictable.
>>>
>>> Which still leaves the questions:
>>>
>>> a) If PG_active and PG_unevictable was cleared, where?
>>
>> For LRU pages, PG_active and PG_unevictable are cleared via __page_cache_release. And for isolated
>> (LRU) pages, PG_active and PG_unevictable should be cleared ourselves?
>>
>>> b) Why is that code block that conditionally clears the flags of any
>>> value and why can't we simply drop it?
>>>
>>
>> To fix the issue, should we clear PG_active and PG_unevictable unconditionally here?
> 
> I wonder if we should simply teach actual freeing code to simply clear
> both flags when freeing an isolated page? IOW, to detect "isolated LRU"
> is getting freed and fixup?

IMHO, clearing both flags are done by the caller indeed. Another example I found when I
read the khugepaged code recently is pasted below:

collapse_file
	...
	page_ref_unfreeze(page, 1);
	ClearPageActive(page);
	ClearPageUnevictable(page);
	unlock_page(page);
	put_page(page);
	index++;
	...

Thanks!

>