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

[Feng Tang]

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.

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 :)

> 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?

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
  */