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

Linus Torvalds
2022-04-14T22:26:26Z

On Thu, Apr 14, 2022 at 12:49 PM Catalin Marinas
<catalin.marinas@arm.com> wrote:
>
> It's a lot worse, ARCH_KMALLOC_MINALIGN is currently 128 bytes on arm64.
> I want to at least get it down to 64 with this series while preserving
> the current kmalloc() semantics.

So here's a thought - maybe we could do the reverse of GFP_DMA, and
add a flag to the places that want small allocations and know they
don't need DMA?

Because even 64 bytes is _huge_ for those small allocations. And 128
bytes is just insane.

Maybe we don't have a ton of them, but I just checked my desktop, and
if my laptop had 17k 8-byte allocation, on my desktop it's currently
sitting at 43k of them. And I've got a lot of 16- and 32-byte ones
too:

kmalloc-32 51529 51712 32 128 1 :
kmalloc-16 45056 45056 16 256 1 :
kmalloc-8 43008 43008 8 512 1 :

Don't ask me what they are. It's probably fairly easy to find out, and
it's probably something silly like sysfs private pointer data or
whatever.

If I did my math right, with a 128-byte minimum allocation, that is
16MB of wasted memory.

Yeah, yeah, I've got 64GB or RAM in this thing, and there are things
that take a lot more memory than the above (mem_map etc), and 64MB is
peanuts at just 0.1% of RAM.

Maybe nobody cares. But I really feel bad about that kind of egregious
waste. The mem_map[] array may be big, it may use 1.5% of the memory I
have, but at least it *does* something.

And it could be that if I have 150k of those smallish allocations, a
server with lots of active users might have millions. Not having
looked at where they come from, maybe that isn't the case, but it
*might* be.

Maybe adding something like a

static int warn_every_1k = 0;
WARN_ON(size < 32 && (1023 & ++warn_every_1k));

to kmalloc() would give us a statistical view of "lots of these small
allocations" thing, and we could add GFP_NODMA to them. There probably
aren't that many places that have those small allocations, and it's
certainly safer to annotate "this is not for DMA" than have the
requirement that all DMA allocations must be marked.

Then just teach kmalloc() something like

align = (gfp_flags & __GFP_NODMA) ? 8 : 128;

on arm.

Hmm?

Linus