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: <18cdbd92-81db-42be-a290-08462759ffe6@126.com>
Date: Tue, 30 Jul 2024 18:01:45 +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 17:58, David Hildenbrand 写道:
> On 30.07.24 11:56, Ge Yang wrote:
>>
>>
>> 在 2024/7/30 17:41, David Hildenbrand 写道:
>>> On 30.07.24 11:36, Ge Yang wrote:
>>>>
>>>>
>>>> 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).
>>>
>>> In which case is folio_activate() called without the PTL on a mapped
>>> page or without a raised refcount?
>>>
>>
>> No such case has been found. But, folio_put() can be run at anytime, so
>> folio_activate() may access a folio with a reference count of 0.
> 
> If you can't find such a case then nothing is broken and no switch to 
> folio_try_get() is required.
> 

Ok, thanks.