linux-kernel - Re: [PATCH AUTOSEL 4.14 72/95] devres: Align data[] to ARCH_KMALLOC

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Date:   Tue, 7 May 2019 07:52:14 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Sasha Levin <sashal@...nel.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Alexey Brodkin <alexey.brodkin@...opsys.com>,
        Alexey Brodkin <abrodkin@...opsys.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        David Laight <David.Laight@...lab.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vineet Gupta <vgupta@...opsys.com>,
        Will Deacon <will.deacon@....com>,
        Sasha Levin <alexander.levin@...rosoft.com>
Subject: Re: [PATCH AUTOSEL 4.14 72/95] devres: Align data[] to
 ARCH_KMALLOC_MINALIGN

On Tue, May 07, 2019 at 01:38:01AM -0400, Sasha Levin wrote:
> From: Alexey Brodkin <alexey.brodkin@...opsys.com>
> 
> [ Upstream commit a66d972465d15b1d89281258805eb8b47d66bd36 ]
> 
> Initially we bumped into problem with 32-bit aligned atomic64_t
> on ARC, see [1]. And then during quite lengthly discussion Peter Z.
> mentioned ARCH_KMALLOC_MINALIGN which IMHO makes perfect sense.
> If allocation is done by plain kmalloc() obtained buffer will be
> ARCH_KMALLOC_MINALIGN aligned and then why buffer obtained via
> devm_kmalloc() should have any other alignment?
> 
> This way we at least get the same behavior for both types of
> allocation.
> 
> [1] http://lists.infradead.org/pipermail/linux-snps-arc/2018-July/004009.html
> [2] http://lists.infradead.org/pipermail/linux-snps-arc/2018-July/004036.html
> 
> Signed-off-by: Alexey Brodkin <abrodkin@...opsys.com>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Geert Uytterhoeven <geert@...ux-m68k.org>
> Cc: David Laight <David.Laight@...LAB.COM>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Vineet Gupta <vgupta@...opsys.com>
> Cc: Will Deacon <will.deacon@....com>
> Cc: Greg KH <greg@...ah.com>
> Cc: <stable@...r.kernel.org> # 4.8+
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Signed-off-by: Sasha Levin <alexander.levin@...rosoft.com>
> ---
>  drivers/base/devres.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/base/devres.c b/drivers/base/devres.c
> index 71d577025285..e43a04a495a3 100644
> --- a/drivers/base/devres.c
> +++ b/drivers/base/devres.c
> @@ -25,8 +25,14 @@ struct devres_node {
>  
>  struct devres {
>  	struct devres_node		node;
> -	/* -- 3 pointers */
> -	unsigned long long		data[];	/* guarantee ull alignment */
> +	/*
> +	 * Some archs want to perform DMA into kmalloc caches
> +	 * and need a guaranteed alignment larger than
> +	 * the alignment of a 64-bit integer.
> +	 * Thus we use ARCH_KMALLOC_MINALIGN here and get exactly the same
> +	 * buffer alignment as if it was allocated by plain kmalloc().
> +	 */
> +	u8 __aligned(ARCH_KMALLOC_MINALIGN) data[];
>  };
>  
>  struct devres_group {

This is not needed in any of the older kernels, despite what the stable@
line said, as it ends up taking a lot of memory up for all other arches.
That's why I only applied it to the one kernel version.  I'm betting
that it will be eventually reverted when people notice it as well :)

So can you please drop it from all of your trees?

thanks,

greg k-h