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: <a8abf253-b1bb-422a-9d3f-d0dd24990617@redhat.com>
Date: Tue, 30 Jul 2024 11:41:27 +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 11:36, 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?

-- 
Cheers,

David / dhildenb