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Date:   Sun, 17 Apr 2022 16:08:22 +0800
From:   Herbert Xu <herbert@gondor.apana.org.au>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        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>,
        Linus Torvalds <torvalds@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>
Subject: Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of
 ARCH_KMALLOC_MINALIGN
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On Fri, Apr 15, 2022 at 01:38:15PM +0200, Ard Biesheuvel wrote:
>
> If this is the case, things are already broken today. We never take
> ARCH_DMA_MINALIGN into account when adding the driver ctx size to the
> overall allocation size.

No it's not broken because kmalloc guarantees alignment.  For
example, if ARCH_DMA_MINALIGN is 128 bytes, then kmalloc will
always return a pointer that's 128-byte aligned.  That guarantees
this object and the next object are on different cache-lines.

If you reduce the kmalloc minimum alignment to 64 bytes, then
the two neighbouring objects can share cache-lines, even if
each object is bigger than 128 bytes (e.g., if they were 192
bytes each).

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