linux-kernel - Re: [PATCH V2] mm/gup: Clear the LRU flag of a page before adding to LRU batch

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Message-ID: <1c5f1582-d6ea-4e27-a966-e6e992cf7c22@126.com>
Date: Tue, 30 Jul 2024 17:36:20 +0800
From: Ge Yang <yangge1116@....com>
To: David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, stable@...r.kernel.org,
 21cnbao@...il.com, baolin.wang@...ux.alibaba.com, liuzixing@...on.cn
Subject: Re: [PATCH V2] mm/gup: Clear the LRU flag of a page before adding to
 LRU batch



在 2024/7/30 15:45, David Hildenbrand 写道:
>>> Looking at this in more detail, I wonder if we can turn that to
>>>
>>> if (!folio_test_clear_lru(folio))
>>>       return;
>>> folio_get(folio);
>>>
>>> In all cases? The caller must hold a reference, so this should be fine.
>>>
>>
>> Seems the caller madvise_free_pte_range(...), calling
>> folio_mark_lazyfree(...), doesn't hold a reference on folio.
>>
> 
> If that would be the case and the folio could get freed concurrently, 
> the folio_get(folio) would be completely broken.
> 
> In madvise_free_pte_range() we hold the PTL, so the folio cannot get 
> freed concurrently.
> 

Right.

> folio_get() is only allowed when we are sure the folio cannot get freed 
> concurrently, because we know there is a reference that cannot go away.
> 
> 

When cpu0 runs folio_activate(), and cpu1 runs folio_put() concurrently, 
a possible bad scenario would like:

cpu0                                           cpu1

                                            folio_put_testzero(folio)
if (!folio_test_clear_lru(folio))// Seems folio shouldn't be accessed 

        return;
folio_get(folio);
                                             __folio_put(folio)
                                             __folio_clear_lru(folio)


Seems we should use folio_try_get(folio) instead of folio_get(folio).