Re: [PATCH -next 1/2] mm/slab: add is_kmalloc_cache() helper macro

[Vlastimil Babka]

On 11/22/22 06:30, Feng Tang wrote:
> On Mon, Nov 21, 2022 at 12:19:38PM -0800, Andrew Morton wrote:
>> On Mon, 21 Nov 2022 21:50:23 +0800 Feng Tang <feng.tang@xxxxxxxxx> wrote:
>> 
>> > +#ifndef CONFIG_SLOB
>> > +#define is_kmalloc_cache(s) ((s)->flags & SLAB_KMALLOC)
>> > +#else
>> > +#define is_kmalloc_cache(s) (false)
>> > +#endif
>> 
>> Could be implemented as a static inline C function, yes?
> 
> Right, I also did try inline function first, and met compilation error: 
> 
> "
> ./include/linux/slab.h: In function ‘is_kmalloc_cache’:
> ./include/linux/slab.h:159:18: error: invalid use of undefined type ‘struct kmem_cache’
>   159 |         return (s->flags & SLAB_KMALLOC);
>       |                  ^~
> "
> 
> The reason is 'struct kmem_cache' definition for slab/slub/slob sit
> separately in slab_def.h, slub_def.h and mm/slab.h, and they are not
> included in this 'include/linux/slab.h'. So I chose the macro way.

You could try mm/slab.h instead, below the slub_def.h includes there.
is_kmalloc_cache(s) shouldn't have random consumers in the kernel anyway.
It's fine if kasan includes it, as it's intertwined with slab a lot anyway.

> Btw, I've worked on some patches related with sl[auo]b recently, and
> really felt the pain when dealing with 3 allocators, on both reading
> code and writing patches. And I really like the idea of fading away
> SLOB as the first step :)

Can't agree more :)

>> If so, that's always best.  For (silly) example, consider the behaviour
>> of
>> 
>> 	x = is_kmalloc_cache(s++);
>> 
>> with and without CONFIG_SLOB.
> 
> Another solution I can think of is putting the implementation into
> slab_common.c, like the below?

The overhead of function call between compilation units (sans LTO) is not
worth it.

> Thanks,
> Feng
> 
> ---
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 067f0e80be9e..e4fcdbfb3477 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -149,6 +149,17 @@
>  
>  struct list_lru;
>  struct mem_cgroup;
> +
> +#ifndef CONFIG_SLOB
> +extern bool is_kmalloc_cache(struct kmem_cache *s);
> +#else
> +static inline bool is_kmalloc_cache(struct kmem_cache *s)
> +{
> +	return false;
> +}
> +#endif
> +
>  /*
>   * struct kmem_cache related prototypes
>   */
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index a5480d67f391..860e804b7c0a 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -77,6 +77,13 @@ __setup_param("slub_merge", slub_merge, setup_slab_merge, 0);
>  __setup("slab_nomerge", setup_slab_nomerge);
>  __setup("slab_merge", setup_slab_merge);
>  
> +#ifndef CONFIG_SLOB
> +bool is_kmalloc_cache(struct kmem_cache *s)
> +{
> +	return (s->flags & SLAB_KMALLOC);
> +}
> +#endif
> +
>  /*
>   * Determine the size of a slab object
>   */