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[PATCH 2/2] mm/mm.h: Write folio->_flags_1 & 0xff as a macro definition

Liu Ye posted 2 patches 10 months, 1 week ago
[PATCH 2/2] mm/mm.h: Write folio->_flags_1 & 0xff as a macro definition
Posted by Liu Ye 10 months, 1 week ago 
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@kylinos.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)
+
 /*
  * 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
 }
 
-- 
2.25.1
Re: [PATCH 2/2] mm/mm.h: Write folio->_flags_1 & 0xff as a macro definition
Posted by David Hildenbrand 10 months, 1 week ago 
On 12.02.25 03:58, 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@kylinos.cn>

I have something different (better) in the works:

https://lore.kernel.org/linux-mm/20240829165627.2256514-3-david@redhat.com/

-- 
Cheers,

David / dhildenb
Re: [PATCH 2/2] mm/mm.h: Write folio->_flags_1 & 0xff as a macro definition
Posted by Matthew Wilcox 10 months, 1 week ago 
On Wed, Feb 12, 2025 at 10:58:43AM +0800, 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.

No.
Re: [PATCH 2/2] mm/mm.h: Write folio->_flags_1 & 0xff as a macro definition
Posted by Dev Jain 10 months, 1 week ago 

On 12/02/25 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@kylinos.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)
> +
>   /*
>    * 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
>   }
>   

Personally I do not think this is improving readability. You are 
introducing one more macro for people to decipher instead of directly 
seeing folio->_flags_1 & 0xff. This is similar to whether to write
if (x) => do_stuff(), or if (x != 0) => do_stuff(). The former is more 
"readable" by convention but the latter makes it easier and obvious to 
understand.
Re: [PATCH 2/2] mm/mm.h: Write folio->_flags_1 & 0xff as a macro definition
Posted by Shivank Garg 10 months, 1 week ago 
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@kylinos.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)


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
>  }
>
Re: [PATCH 2/2] mm/mm.h: Write folio->_flags_1 & 0xff as a macro definition
Posted by liuye 10 months, 1 week ago 

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@kylinos.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
>>  }
>>  
>

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