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

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Date:   Fri, 15 Apr 2022 18:45:47 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        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 Fri, Apr 15, 2022 at 12:22:27PM +0200, Ard Biesheuvel wrote:
>
> Subsequent objects are owned by the driver, and it is the
> responsibility of the driver not to modify the fields while it is also
> mapped for DMA (and we have had issues in the past where drivers
> violated this rule). So as long as ARCH_KMALLOC_ALIGN guarantees
> actual DMA minimum alignment for both the start and the end, we
> shouldn't need any explicit padding at the end.

I don't understand why this is guaranteed.  The driver context
size is arbitrary so it could end in the middle of a cacheline.
The slab allocator could well lay it out so that the next kmalloc
object starts right after the end of the context, in which case
they would share a cache-line.

The next kmalloc object could be (and in fact is likely to be)
of the same type.

Previously this wasn't possible because kmalloc guaranteed
alignment.

> I'l do a scan of drivers/crypto to figure out how much we are relying
> on this to begin with.

Thanks,
-- 
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