Re: [PATCH v1 26] mm allow reuse of the lower 16 bit of the page type with an actual type

[David Hildenbrand <david@redhat.com>]

On 29.05.24 17:55, wang wei wrote:
> ---
>> As long as the owner sets a page type first, we can allow reuse of the
>> lower 18 bit: sufficient to store an offset into a 64 KiB page, which
>> is the maximum base page size in *common* configurations (ignoring the
>> 256 KiB variant). Restrict it to the head page.
>>
>> We'll use that for zsmalloc next, to set a proper type while still
>> reusing that field to store information (offset into a base page) that
>> cannot go elsewhere for now.
>>
>> Fear of running out of bits for storing the actual type? Actually, we
>> don't need one bit per type, we could store a single value instead.
>> Further, we could likely limit PAGE_TYPE_BASE to a single (highest) bit.
>>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>   include/linux/mm_types.h   |  5 +++++
>>   include/linux/page-flags.h | 20 ++++++++++++--------
>>   2 files changed, 17 insertions(+), 8 deletions(-)
>>
>> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
>> index 6b2aeba792c4..598cfedbbfa0 100644
>> --- a/include/linux/mm_types.h
>> +++ b/include/linux/mm_types.h
>> @@ -157,6 +157,11 @@ struct page {
>>   		 *
>>   		 * See page-flags.h for a list of page types which are currently
>>   		 * stored here.
>> +		 *
>> +		 * Owners of typed folios may reuse the lower 16 bit of the
>> +		 * head page page_type field after setting the page type,
>> +		 * but must reset these 16 bit to -1 before clearing the
>> +		 * page type.
>>   		 */
>>   		unsigned int page_type;
>>   
>> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
>> index 104078afe0b1..b43e380ffa0b 100644
>> --- a/include/linux/page-flags.h
>> +++ b/include/linux/page-flags.h
>> @@ -945,14 +945,18 @@ PAGEFLAG_FALSE(HasHWPoisoned, has_hwpoisoned)
>>    */
>>   
>>   #define PAGE_TYPE_BASE	0xf0000000
>> -/* Reserve		0x0000007f to catch underflows of _mapcount */
>> -#define PAGE_MAPCOUNT_RESERVE	-128
>> -#define PG_buddy	0x00000080
>> -#define PG_offline	0x00000100
>> -#define PG_table	0x00000200
>> -#define PG_guard	0x00000400
>> -#define PG_hugetlb	0x00000800
>> -#define PG_slab		0x00001000
>> +/*
>> + * Reserve		0x0000ffff to catch underflows of _mapcount and
>> + * allow owners that set a type to reuse the lower 16 bit for their own
>> + * purposes.
>> + */
>> +#define PAGE_MAPCOUNT_RESERVE	-65536
>> +#define PG_buddy	0x00010000
>> +#define PG_offline	0x00020000
>> +#define PG_table	0x00040000
>> +#define PG_guard	0x00080000
>> +#define PG_hugetlb	0x00100800
> 
> Every PG_XX occupies one bit in my understanding.   But why PG_hugetlb occupies two bits?

Because it's wrong (although not harmful). Same issue in v2, fat fingers.

Thanks for pointing that out!

-- 
Cheers,

David / dhildenb