Re: [PATCH] mm: move PG_slab flag to page_type

[Hyeonggon Yoo]

On Mon, Sep 19, 2022 at 09:57:08PM +0900, Hyeonggon Yoo wrote:
> For now, only SLAB uses _mapcount field as a number of active objects in
> a slab, and other slab allocators do not use it. As 16 bits are enough
> for that, use remaining 16 bits of _mapcount as page_type even when
> SLAB is used. And then move PG_slab flag to page_type!
> 
> Note that page_type is always placed in upper 16 bits of _mapcount to
> avoid confusing normal _mapcount as page_type. As underflow (actually
> I mean, yeah, overflow) is not a concern anymore, use more lower bits
> except bit zero.
> 
> Add more folio helpers for PAGE_TYPE_OPS() not to break existing
> slab implementations.
> 
> Remove PG_slab check from PAGE_FLAGS_CHECK_AT_FREE. buddy will still
> check if _mapcount is properly set at free.
> 
> Exclude PG_slab from hwpoison and show_page_flags() for now.
> 
> Note that with this patch, page_mapped() and folio_mapped() always return
> false for slab page.
> 

[...]

Hi. a silly mistake:

> 
>  include/linux/mm_types.h       | 22 +++++++--
>  include/linux/page-flags.h     | 83 ++++++++++++++++++++++++++--------
>  include/trace/events/mmflags.h |  1 -
>  mm/memory-failure.c            |  8 ----
>  mm/slab.h                      | 11 ++++-
>  5 files changed, 92 insertions(+), 33 deletions(-)
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index cf97f3884fda..4b217c6fbe1f 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -193,12 +193,24 @@ struct page {
>  		atomic_t _mapcount;
>  
>  		/*
> -		 * If the page is neither PageSlab nor mappable to userspace,
> -		 * the value stored here may help determine what this page
> -		 * is used for.  See page-flags.h for a list of page types
> -		 * which are currently stored here.
> +		 * If the page is not mappable to userspace, the value
> +		 * stored here may help determine what this page is used for.
> +		 * See page-flags.h for a list of page types which are currently
> +		 * stored here.
>  		 */
> -		unsigned int page_type;
> +		struct {
> +			/*
> +			 * Always place page_type in
> +			 * upper 16 bits of _mapcount
> +			 */
> +#ifdef CPU_BIG_ENDIAN

s/CPU_BIG_ENDIAN/CONFIG_CPU_BIG_ENDIAN/g

> +			__u16 page_type;
> +			__u16 active;
> +#else
> +			__u16 active;
> +			__u16 page_type;
> +#endif
> +		};
>  	};
>  
>  	/* Usage count. *DO NOT USE DIRECTLY*. See page_ref.h */

[...]

> diff --git a/mm/slab.h b/mm/slab.h
> index 985820b9069b..a5273e189265 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -20,7 +20,16 @@ struct slab {
>  		};
>  		struct rcu_head rcu_head;
>  	};
> -	unsigned int active;
> +	struct {
> +		/* always place page_type in upper 16 bits of _mapcount */
> +#ifdef CPU_BIG_ENDIAN

same here.

> +		__u16 page_type;
> +		__u16 active;
> +#else
> +		__u16 active;
> +		__u16 page_type;
> +#endif
> +	};
>  
>  #elif defined(CONFIG_SLUB)
>  
> -- 
> 2.32.0
> 

-- 
Thanks,
Hyeonggon