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: <9e018975-8a80-46a6-ab38-f3e2945c8878@redhat.com>
Date: Tue, 30 Jul 2024 09:45:15 +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

>> 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.

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.


-- 
Cheers,

David / dhildenb