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    SubjectRe: [PATCH -next 1/2] mm/slab: add is_kmalloc_cache() helper macro
    On Wed, Nov 23, 2022 at 10:21:03AM +0100, Vlastimil Babka wrote:
    > 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@intel.com> 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.

    Good suggestion! thanks! This can address Andrew's concern and also
    avoid extra cost.

    And yes, besides sanity code like kasan/kfence, rare code will care
    whether other kmem_cache is a kmalloc cache or not. And kasan code
    already includes "../slab.h".

    > > 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.

    Yes. Will send out the v2 patches.

    Thanks,
    Feng

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