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

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Date:   Sun, 17 Apr 2022 09:38:40 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Ard Biesheuvel <ardb@...nel.org>, 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>,
        Linus Torvalds <torvalds@...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 Sun, Apr 17, 2022 at 04:11:22PM +0800, Herbert Xu wrote:
> On Fri, Apr 15, 2022 at 01:31:32PM +0100, Catalin Marinas wrote:
> >
> > This needs a clarification. For the above structure, kmalloc() will
> > return a 128-byte aligned pointer since sizeof(x) is a multiple of 128.
> > The potential problem is if you have something like:
> > 
> > 	kmalloc(sizeof(struct x) + 64);
> > 
> > The above could end up as a kmalloc(192) which is available with an
> > ARCH_KMALLOC_MINALIGN of 64. If that's a real use-case, I can change the
> > slab patch to not create the 192 (or 48 if we go for an even smaller
> > ARCH_KMALLOC_MINALIGN) caches and we'd always have ARCH_DMA_MINALIGN
> > guarantee if the structure itself is correctly aligned. No lying to the
> > compiler.
> 
> Yes I suppose that should work:
> 
> 1) Enlarge each crypto API object so that they're >= 128 bytes;

I don't think we need to do anything here. A structure like:

struct x {
	char y;
	char z[] CRYPTO_MINALIGN_ATTR;
};

is already of size 128. Without CRYPTO_MINALIGN_ATTR, its size would be
1 but otherwise the whole structure inherits the alignment of its
member and this translates into an aligned size.

> 2) Modify kmalloc so that for sizes >= 128 bytes they're padded
> to multiples of 128.

This doesn't look like a hack, we want to honour the power of 2
alignments.

-- 
Catalin