linux-kernel - Re: [PATCH 2/2] mm/mm.h: Write folio->_flags

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Message-ID: <ea27222c-1306-45d5-ad55-6d7780ada06b@kylinos.cn>
Date: Wed, 12 Feb 2025 15:11:47 +0800
From: liuye <liuye@...inos.cn>
To: Shivank Garg <shivankg@....com>, brauner@...nel.org, dhowells@...hat.com,
 akpm@...ux-foundation.org
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 2/2] mm/mm.h: Write folio->_flags_1 & 0xff as a macro
 definition



在 2025/2/12 13:40, Shivank Garg 写道:
> On 2/12/2025 8:28 AM, Liu Ye wrote:
>> There are multiple locations in mm.h where (folio->_flags_1 & 0xff) is
>> used. Write it as a macro definition to improve the readability and
>> maintainability of the code.
>>
>> Signed-off-by: Liu Ye <liuye@...inos.cn>
>> ---
>>  include/linux/mm.h | 10 ++++++----
>>  1 file changed, 6 insertions(+), 4 deletions(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 7b1068ddcbb7..750e75f45557 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -1098,6 +1098,8 @@ int vma_is_stack_for_current(struct vm_area_struct *vma);
>>  struct mmu_gather;
>>  struct inode;
>>  
>> +#define FOLIO_ORDER(folio) ((folio)->_flags_1 & 0xff)
> 
> This folio order calculation is only valid for !large folios.
> When it's a single page (not a large folio), the memory is interpreted as struct page.
> 
> struct folio {
> ...
>         union {
>                 struct {
>                         unsigned long _flags_1;
>                         unsigned long _head_1;
>         /* public: */
>                         atomic_t _large_mapcount;
>                         atomic_t _entire_mapcount;
>                         atomic_t _nr_pages_mapped;
>                         atomic_t _pincount;
> #ifdef CONFIG_64BIT
>                         unsigned int _folio_nr_pages;
> #endif
>         /* private: the union with struct page is transitional */
>                 };
>                 struct page __page_1;
>         };
> ...
> }
> 
> I feel this to be risky, considering someone may directly use FOLIO_ORDER() macro
> without folio_test_large() check.
> 
> Correct macro should look like:
> 
> #define FOLIO_ORDER(folio) (folio_test_large(folio) ? ((folio)->_flags_1 & 0xff) : 0)
> 

Yes, this is safer.
At present, the positions using FOLIO-ORDER have been checked using folio_test_1arge or
test-bit (PG_cead,&folio ->flags), and these positions may need to be simplified.

> 
> Thanks,
> Shivank
>> +
>>  /*
>>   * compound_order() can be called without holding a reference, which means
>>   * that niceties like page_folio() don't work.  These callers should be
>> @@ -1111,7 +1113,7 @@ static inline unsigned int compound_order(struct page *page)
>>  
>>  	if (!test_bit(PG_head, &folio->flags))
>>  		return 0;
>> -	return folio->_flags_1 & 0xff;
>> +	return FOLIO_ORDER(folio);
>>  }
>>  
>>  /**
>> @@ -1127,7 +1129,7 @@ static inline unsigned int folio_order(const struct folio *folio)
>>  {
>>  	if (!folio_test_large(folio))
>>  		return 0;
>> -	return folio->_flags_1 & 0xff;
>> +	return FOLIO_ORDER(folio);
>>  }
>>  
>>  #include <linux/huge_mm.h>
>> @@ -2061,7 +2063,7 @@ static inline long folio_nr_pages(const struct folio *folio)
>>  #ifdef CONFIG_64BIT
>>  	return folio->_folio_nr_pages;
>>  #else
>> -	return 1L << (folio->_flags_1 & 0xff);
>> +	return 1L << FOLIO_ORDER(folio);
>>  #endif
>>  }
>>  
>> @@ -2086,7 +2088,7 @@ static inline unsigned long compound_nr(struct page *page)
>>  #ifdef CONFIG_64BIT
>>  	return folio->_folio_nr_pages;
>>  #else
>> -	return 1L << (folio->_flags_1 & 0xff);
>> +	return 1L << FOLIO_ORDER(folio);
>>  #endif
>>  }
>>  
>