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Date:   Sun, 17 Apr 2022 09:31:09 +0100
From:   Catalin Marinas <catalin.marinas@arm.com>
To:     Herbert Xu <herbert@gondor.apana.org.au>
Cc:     Ard Biesheuvel <ardb@kernel.org>, 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 Sun, Apr 17, 2022 at 04:08:22PM +0800, Herbert Xu wrote:
> 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).

Not with my series, the non-sharing of cache lines is preserved.
kmalloc() still returns objects aligned to a cache-line.
ARCH_DMA_MINALIGN was chosen as the cover-all value for all SoCs
supported but I want to reduce the kmalloc() alignment to a cache line
size if a platform has a cache line smaller than ARCH_DMA_MINALIGN (most
arm64 SoCs have a cache line of 64 bytes rather than 128).

-- 
Catalin