Re: [PATCH v3 1/2] mm/memcg: alignment memcg_data define condition

[Vlastimil Babka <vbabka@suse.cz>]

On 7/10/24 7:43 AM, alexs@kernel.org wrote:
> From: "Alex Shi (Tencent)" <alexs@kernel.org>
> 
> commit 21c690a349ba ("mm: introduce slabobj_ext to support slab object
> extensions") changed the folio/page->memcg_data define condition from
> MEMCG to SLAB_OBJ_EXT. And selected SLAB_OBJ_EXT for MEMCG, just for
> SLAB_MATCH(memcg_data, obj_exts), even no other relationship between them.
> 
> Above action make memcg_data exposed and include SLAB_OBJ_EXT for
> !MEMCG. That's incorrect in logcial and pay on code size.
> 
> As Vlastimil Babka suggested, let's add _unused_slab_obj_ext for
> SLAB_MATCH for slab.obj_exts while !MEMCG. That could resolve the match
> issue, clean up the feature logical. And decouple the SLAB_OBJ_EXT from
> MEMCG in next patch.
> 
> Signed-off-by: Alex Shi (Tencent) <alexs@kernel.org>
> Cc: Randy Dunlap <rdunlap@infradead.org>
> Cc: Yoann Congal <yoann.congal@smile.fr>
> Cc: Masahiro Yamada <masahiroy@kernel.org>
> Cc: Petr Mladek <pmladek@suse.com>
> Cc: Suren Baghdasaryan <surenb@google.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> ---
> v1->v3: take Vlastimil's suggestion and move SLAB_OBJ_EXT/MEMCG decouple
> to 2nd patch.
> ---
>  include/linux/mm_types.h | 8 ++++++--
>  mm/slab.h                | 4 ++++
>  2 files changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index ef09c4eef6d3..4ac3abc673d3 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -180,8 +180,10 @@ struct page {
>  	/* Usage count. *DO NOT USE DIRECTLY*. See page_ref.h */
>  	atomic_t _refcount;
>  
> -#ifdef CONFIG_SLAB_OBJ_EXT
> +#ifdef CONFIG_MEMCG
>  	unsigned long memcg_data;
> +#elif defined(CONFIG_SLAB_OBJ_EXT)
> +	unsigned long _unused_slab_obj_ext;
>  #endif
>  
>  	/*
> @@ -343,8 +345,10 @@ struct folio {
>  			};
>  			atomic_t _mapcount;
>  			atomic_t _refcount;
> -#ifdef CONFIG_SLAB_OBJ_EXT
> +#ifdef CONFIG_MEMCG
>  			unsigned long memcg_data;
> +#elif defined(CONFIG_SLAB_OBJ_EXT)
> +			unsigned long _unused_slab_obj_ext;
>  #endif
>  #if defined(WANT_PAGE_VIRTUAL)
>  			void *virtual;
> diff --git a/mm/slab.h b/mm/slab.h
> index 3586e6183224..8ffdd4f315f8 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -98,7 +98,11 @@ SLAB_MATCH(flags, __page_flags);
>  SLAB_MATCH(compound_head, slab_cache);	/* Ensure bit 0 is clear */
>  SLAB_MATCH(_refcount, __page_refcount);
>  #ifdef CONFIG_SLAB_OBJ_EXT
> +#ifdef CONFIG_MEMCG
>  SLAB_MATCH(memcg_data, obj_exts);
> +#else
> +SLAB_MATCH(_unused_slab_obj_ext, obj_exts);
> +#endif
>  #endif

Why not also #ifdef / #elif like above, instead of this nesting?

>  #undef SLAB_MATCH
>  static_assert(sizeof(struct slab) <= sizeof(struct page));