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

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Message-ID: <YlvTNQGh+MfZFWKW@gondor.apana.org.au>
Date:   Sun, 17 Apr 2022 16:43:33 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Catalin Marinas <catalin.marinas@....com>
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 09:38:40AM +0100, Catalin Marinas wrote:
> 
> 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.

No we should not lie to the compiler, we have code elsewhere
that uses the alignment to compute the amount of extra padding
needed to create greater padding.  If CRYPTO_MINALIGN is misleading
then that calculation will fall apart.

So keep CRYPTO_MINALIGN at whatever alignment you lower kmalloc
to, and then add the padding you need to separate the Crypto API
bits from the context.  In fact, that is exactly what cra_alignmask
is supposed to do.

Sure we currently limit the maximum alignment to 64 bytes because
of stack usage but we can certainly look into increasing it as
that's what you're doing here anyway.

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt