Re: [PATCH v5 2/4] slab: Introduce kmalloc_obj() and family

[Linus Torvalds <torvalds@linux-foundation.org>]

Btw, I realize that we don't have a good way to do the alignment with
the current kmalloc() interface (we do for some of the vmalloc
interfaces).

So for now, it should just have some static build-time warning if the
type of the object we allocate has a bigger alignment than the
guaranteed slab allocation alignment (ARCH_KMALLOC_MINALIGN or
whatever).

And I really think the first version should do the minimal thing that
actually matters, and strive to deal with the simple cases. The main
things that matter are

 - the return type should be a proper pointer type (so that you get
warnings for mis-uses, but also so that you can use automatic typing)

 - making the 'sizeof()' match the type

so honestly, I think 99% of the gain would come from something fairly
simple like

    #define kmalloc_verify(type) \
        BUILD_BUG_ON_ZERO(__alignof__(type) > ARCH_KMALLOC_MINALIGN)

    #define kmalloc_size(type) \
        (sizeof(type) + kmalloc_verify(type))

    #define allocator(name, type, size, ...) \
        (typeof(type) *)name(size, __VA_ARGS__)

    #define kmalloc_obj(type, gfp) \
        allocator(kmalloc, type, kmalloc_size(type), gfp)
    #define kzalloc_obj(type, gfp) \
        allocator(kzalloc, type, kmalloc_size(type), gfp)
    #define kzalloc_struct(type, member, count, gfp) \
        allocator(kzalloc, type, struct_size_t(typeof(type), member,
count), gfp)

The above macros are entirely untested. But they are simple enough
that even if they are buggy and I miscounted the parentheses or used
the wrong name somewhere, I think the idea is clear. No?

(And I made that "allocator()" macro use __VA_ARGS__ because
kzalloc_node() and friends would want that, but I think it's starting
to hit diminishing returns at that point)

Hmm?

               Linus