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Date:   Sat, 16 Apr 2022 10:42:04 +0100
From:   Catalin Marinas <catalin.marinas@arm.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Ard Biesheuvel <ardb@kernel.org>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        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>,
        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 Thu, Apr 14, 2022 at 03:25:59PM -0700, Linus Torvalds wrote:
> On Thu, Apr 14, 2022 at 12:49 PM Catalin Marinas
> <catalin.marinas@arm.com> wrote:
> > It's a lot worse, ARCH_KMALLOC_MINALIGN is currently 128 bytes on arm64.
> > I want to at least get it down to 64 with this series while preserving
> > the current kmalloc() semantics.
> 
> So here's a thought - maybe we could do the reverse of GFP_DMA, and
> add a flag to the places that want small allocations and know they
> don't need DMA?

I wonder whether that's a lot more churn than trying to identify places
where a small kmalloc()'ed buffer is passed to the DMA API. DMA into
kmalloc() buffers should be a small fraction of the total kmalloc()
uses.

For kmem_cache we have the SLAB_HWCACHE_ALIGN flag. We can add a similar
GFP_ flag as that's what we care about for DMA safety. It doesn't even
need to force the alignment to ARCH_DMA_MINALIGN but just
cache_line_size() (typically 64 on arm64 while ARCH_DMA_MINALIGN is 128
for about three platforms that have this requirement).

Functions like dma_map_single() can be made to track down the origin of
the buffer when size < cache_line_size() and warn if the slab is not
correctly aligned.

-- 
Catalin