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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 14 Oct 2022 13:44:25 -0700
Message-ID: <CAHk-=wjgaWEsObv2SO-xa6g6ncj53q8tPpM3cgJGr+wtWknVyA@mail.gmail.com>
Subject: Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of ARCH_KMALLOC_MINALIGN
To:     Saravana Kannan <saravanak@google.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Isaac Manjarres <isaacmanjarres@google.com>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Ard Biesheuvel <ardb@kernel.org>,
        Will Deacon <will@kernel.org>, Marc Zyngier <maz@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "David S. Miller" <davem@davemloft.net>, kernel-team@android.com
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On Fri, Oct 14, 2022 at 1:24 PM Saravana Kannan <saravanak@google.com> wrote:
>
> Agreed. Even allowing a 64-byte kmalloc cache on a system with a
> 64-byte cacheline size saves quite a bit of memory.

Well, the *really* trivial thing to do is to just say "if the platform
is DMA coherent, just allow any size kmalloc cache". And just
consciously leave the broken garbage behind.

Because it's not just 64-byte kmalloc. It's things like 'avtab_node'
that is 24 bytes, and that on my system currently uses about 3MB of
memory simply because there's a _lot_ of them.

I've got 1.8MB in "kmalloc-32" too, and about 1MB in "kamlloc-16", fwiw. That's

Yeah, yeah, this is on a 64GB machine and so none of that matters (and
some of these things are very much "scales with memory", but these
small allocations aren't actually all that unusual.

And yes, the above is obviously on my x86-64 machine.

My arm64 laptop doesn't have the small kmallocs, and as a result the
"kmalloc-128" has 633 _thousand_ entries, and takes up 77MB of RAM on
my 16GB laptop. I'm assuming (but have no proof) more than 50% of that
is just wasted memory.

I suspect that DMA is cache coherent on this thing, and that wasted
space is just *stupid* wasted space, but hey, I don't actually know. I
just use the Asahi people's patches.

               Linus