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: <d41865b4-d6fa-49ba-890a-921eefad27dd@redhat.com>
Date: Tue, 30 Jul 2024 11:58:31 +0200
From: David Hildenbrand <david@...hat.com>
To: Ge Yang <yangge1116@....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

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.

-- 
Cheers,

David / dhildenb