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

[Alex Shi]

On 7/11/24 4:13 PM, Vlastimil Babka wrote:
> On 7/10/24 7:43 AM, alexs@xxxxxxxxxx wrote:
>> From: "Alex Shi (Tencent)" <alexs@xxxxxxxxxx>
>>
>> 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@xxxxxxxxxx>
>> Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
>> Cc: Yoann Congal <yoann.congal@xxxxxxxx>
>> Cc: Masahiro Yamada <masahiroy@xxxxxxxxxx>
>> Cc: Petr Mladek <pmladek@xxxxxxxx>
>> Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
>> Cc: Vlastimil Babka <vbabka@xxxxxxx>
>> ---
>> 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?

Uh, it works too if MEMCG/SLAB_OBJ_EXT decoupled.
but right, it could be written with #ifdef/#elif.

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