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Message-ID: <736282e2-e17f-4434-9009-8252055451c5@redhat.com>
Date: Wed, 1 May 2024 17:54:20 +0200
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Subject: Re: [PATCH v5] mm/rmap: do not add fully unmapped large folio to
 deferred split list
To: Zi Yan <ziy@nvidia.com>, Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
 "Matthew Wilcox (Oracle)" <willy@infradead.org>,
 Yang Shi <shy828301@gmail.com>, Ryan Roberts <ryan.roberts@arm.com>,
 Barry Song <21cnbao@gmail.com>, Lance Yang <ioworker0@gmail.com>,
 linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org
References: <20240426190253.541419-1-zi.yan@sent.com>
 <ZjJCdodEf4CBa1N7@li-008a6a4c-3549-11b2-a85c-c5cc2836eea2.ibm.com>
 <5FAD7FB6-4AC0-4427-B56E-4942A95DB68C@nvidia.com>
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From: David Hildenbrand <david@redhat.com>
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In-Reply-To: <5FAD7FB6-4AC0-4427-B56E-4942A95DB68C@nvidia.com>
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On 01.05.24 15:38, Zi Yan wrote:
> On 1 May 2024, at 9:24, Alexander Gordeev wrote:
> 
>> On Fri, Apr 26, 2024 at 03:02:53PM -0400, Zi Yan wrote:
>>
>> Hi Zi,
>>
>> It increasingly looks like this commit is crashing on s390 since
>> 2024-04-30 in linux-next. If I do not miss something - since it
>> was included in mm-everything.
>>
>>> @@ -1553,9 +1558,10 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>>>   		 * page of the folio is unmapped and at least one page
>>>   		 * is still mapped.
>>>   		 */
>>> -		if (folio_test_large(folio) && folio_test_anon(folio))
>>> -			if (level == RMAP_LEVEL_PTE || nr < nr_pmdmapped)
>>> -				deferred_split_folio(folio);
>>> +		if (folio_test_anon(folio) &&
>>> +		    list_empty(&folio->_deferred_list) &&
>>
>> An attempt to reference folio->_deferred_list causes the crash below.
> 
> So if you remove this line, the crash no longer happens? It looks strange to
> me that referencing a anonymous folio's _deferred_list would cause a crash.
> Hmm, unless the folio is order-0.
> 
> Can you try the patch below and see if it fixes the crash? It moves partially_mapped
> ahead to exclude order-0 folios.
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 087a79f1f611..2d27c92bb6d5 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1557,9 +1557,8 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>                   * page of the folio is unmapped and at least one page
>                   * is still mapped.
>                   */
> -               if (folio_test_anon(folio) &&
> -                   list_empty(&folio->_deferred_list) &&
> -                   partially_mapped)
> +               if (folio_test_anon(folio) && partially_mapped &&
> +                   list_empty(&folio->_deferred_list))
>                          deferred_split_folio(folio);

Yes, that should fix it and is the right thing to do. For small folios, 
partially_mapped will always be false.

-- 
Cheers,

David / dhildenb