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

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Date:   Fri, 15 Jul 2022 15:23:25 -0700
From:   Isaac Manjarres <isaacmanjarres@...gle.com>
To:     Catalin Marinas <catalin.marinas@....com>,
        Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        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>,
        Saravana Kannan <saravanak@...gle.com>, kernel-team@...roid.com
Subject: Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of
 ARCH_KMALLOC_MINALIGN

On Sun, Apr 17, 2022 at 05:29:01PM +0100, Catalin Marinas wrote:
> On Sun, Apr 17, 2022 at 04:43:33PM +0800, Herbert Xu wrote:
> > 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 won't if we ensure that a structure with sizeof() >= 128 is aligned
> to 128.
> 
Right. kmalloc() should return a 128 byte aligned pointer as long as
the size of the allocation is >= 128 bytes, and the kmalloc-192 cache
isn't present. So, the current behavior that crypto is relying on
wouldn't change, so I agree with Catalin that we wouldn't be lying to
the compiler if we move forward with getting rid of kmalloc-192.

FWIW, I did a comparison on my machine with and without kmalloc-192, and
the amount of memory usage that increased from allocations being redirected to
kmalloc-256 was about 0.4-0.5 MB, which doesn't seem too bad.

> > 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.
I don't think it would be misleading. If all of your allocations
are >= CRYPTO_MINALIGN == ARCH_DMA_MINALIGN in size, and
kmalloc()--with kmalloc-192 removed--returns buffers that are aligned to a
power of 2, and are big enough to accomodate your allocation, then wouldn't
they always be CYRPTO_MINALIGN'ed, so your calculation would still be fine?

--Isaac