linux-kernel - Re: [PATCH linux-next] mm: huge_memory: fix misused mapping_large_folio

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Date: Wed, 5 Jun 2024 09:25:37 +0200
From: David Hildenbrand <david@...hat.com>
To: ran xiaokai <ranxiaokai627@....com>
Cc: akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, mhocko@...nel.org, v-songbaohua@...o.com,
 ran.xiaokai@....com.cn, xu.xin16@....com.cn, yang.yang29@....com.cn,
 ziy@...dia.com
Subject: Re: [PATCH linux-next] mm: huge_memory: fix misused
 mapping_large_folio_support() for anon folios

On 05.06.24 04:20, ran xiaokai wrote:
>> On 04.06.24 07:47, xu.xin16@....com.cn wrote:
>>> From: Ran Xiaokai <ran.xiaokai@....com.cn>
>>>
>>> When I did a large folios split test, a WARNING
>>> "[ 5059.122759][  T166] Cannot split file folio to non-0 order"
>>> was triggered. But my test cases are only for anonmous folios.
>>> while mapping_large_folio_support() is only reasonable for page
>>> cache folios.
>>
>> Agreed.
>>
>> I wonder if mapping_large_folio_support() should either
>>
>> a) Complain if used for anon folios, so we can detect the wrong use more
>> easily. (VM_WARN_ON_ONCE())
> 
>> b) Return "true" for anonymous mappings, although that's more debatable.
>>
> 
> Hi, David,
> Thanks for the review.
> I think a) is better.
> But we have to add a new parameter "folio" to mapping_large_folio_support(), right ?
> something like mapping_large_folio_support(struct address_space *mapping, struct folio *folio) ?
> But in the __filemap_get_folio() path,
> 
> __filemap_get_folio()
>    no_page:
>      ....
>      if (!mapping_large_folio_support(mapping))
> 
> the folio is not allocated yet, yes ?
> Or do you mean there is some other way to do this ?

If we really pass unmodified folio->mapping, you can do what 
folio_test_anon() would and make sure PAGE_MAPPING_ANON is not set.

-- 
Cheers,

David / dhildenb