Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of ARCH_KMALLOC_MINALIGN

Linus Torvalds
2022-04-20T19:34:08Z

On Wed, Apr 20, 2022 at 12:08 PM Catalin Marinas
<catalin.marinas@arm.com> wrote:
>
> With kstrdup() modified to pass __GFP_PACKED (as per the last hunk in
> the diff below), I get just after boot:
>
> kmalloc-128 8966 9056 128 32
> kmalloc-96 0 0 96 42
> kmalloc-64 192 192 64 64
> kmalloc-32 768 768 32 128
> kmalloc-16 2048 2048 16 256
> kmalloc-8 2560 2560 8 512
>
> So that's probably the simplest approach and using the ftrace histogram
> we can add the flag to more places.

I agree that this seems to be the safest thing to do, and maybe
__GFP_PACKED is a better flag name than __GFP_NODMA.

That said, It worries me a bit in that to me "PACKED" implies "no
alignment at all". And I could _easily_ see people still wanting to do
8-byte allocations that have 8-byte alignment because it's some kind
of private pointer thing or whatever.

For "kstrdup()", a flag like __GFP_PACKED makes 100% sense, since it
literally wants byte alignment.

But what about those "random small structures" cases?

Anyway, I'm perfectly happy calling it __GFP_PACKED, but at a minimum
document that "packed" in this case still means "__alignof__(unsigned
long long)" or something like that?

Linus