linux-kernel - Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of ARCH_KMALLOC

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Message-ID: <CAHk-=widM2Zt2BK3dwpUG8SzPG7_oG7t7ZvDoQmvDWDWwkB7TA@mail.gmail.com>
Date:   Wed, 20 Apr 2022 12:33:39 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Ard Biesheuvel <ardb@...nel.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of ARCH_KMALLOC_MINALIGN

On Wed, Apr 20, 2022 at 12:08 PM Catalin Marinas
<catalin.marinas@....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