Re: Sparse not supporting kzalloc_obj() and friends

On Mon, 23 Feb 2026 at 08:19, Linus Torvalds
<[email protected]> wrote:
>
> Argh. The culprit is the use of __VA_OPT__, which apparently sparse
> never got taught.
>    [...]
> Ho humm. I guess I should look at the sparse pre-processor code,
> unless somebody already implemented __VA_OPT__ somewhere?

Actually, it looks like Al at least *looked* at doing __VA_OPT__ back
at the end of last year. In fact, just a couple of months ago I said
"I'm not sure it buys us much", but that was when I thought it was
just a standard versions of the gcc ## thing, and before I realized
how much more convenient it actually is to use.

Al? You know the pre-process.c file a *lot* better than I do, even
ignoring the fact that it's been fifteen years since I touched it. And
you presumably already had some thoughts on doing __VA_OPT__?

That said, I can also just make default_gfp() use ## after all. I
_had_ grepped for __VA_OPT__ in the kernel and decided "we already use
it", but clearly it's causing some issues.

So instead of <linux/gfp.h> doing

  #define __default_gfp(a,...) a
  #define default_gfp(...) __default_gfp(__VA_ARGS__ __VA_OPT__(,) GFP_KERNEL)

which I think is fairly straightforward and the "natural" way to do
things, the kernel can just do

  #define __default_gfp(a,b,...) b
  #define default_gfp(...) __default_gfp(,##__VA_ARGS__,GFP_KERNEL)

instead, with a dummy empty argument to make the gcc ## trick work.
It's smaller and denser, so it's actually superior in that sense, even
if it feels a bit less natural than the __VA_OPT__ model.

Ricardo, can you verify that that trick works for you?

Al, comments?

             Linus