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From:   Alexey Brodkin <Alexey.Brodkin@synopsys.com>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Sasha Levin <sashal@kernel.org>
CC:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        David Laight <David.Laight@aculab.com>,
        "Peter Zijlstra" <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Vineet Gupta <Vineet.Gupta1@synopsys.com>,
        Will Deacon <will.deacon@arm.com>,
        Sasha Levin <alexander.levin@microsoft.com>
Subject: RE: [PATCH AUTOSEL 4.14 72/95] devres: Align data[] to
 ARCH_KMALLOC_MINALIGN
Thread-Topic: [PATCH AUTOSEL 4.14 72/95] devres: Align data[] to
 ARCH_KMALLOC_MINALIGN
Thread-Index: AQHVBJd3onwrWwkS2UueAl57c5ZJEaZfKNsAgAATBJA=
Date:   Tue, 7 May 2019 07:04:13 +0000
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References: <20190507053826.31622-1-sashal@kernel.org>
 <20190507053826.31622-72-sashal@kernel.org>
 <20190507055214.GA17986@kroah.com>
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Hi Greg,

> -----Original Message-----
> From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Sent: Tuesday, May 7, 2019 8:52 AM
> To: Sasha Levin <sashal@kernel.org>
> Cc: linux-kernel@vger.kernel.org; stable@vger.kernel.org; Alexey Brodkin =
<abrodkin@synopsys.com>;
> Alexey Brodkin <abrodkin@synopsys.com>; Geert Uytterhoeven <geert@linux-m=
68k.org>; David Laight
> <David.Laight@aculab.com>; Peter Zijlstra <peterz@infradead.org>; Thomas =
Gleixner
> <tglx@linutronix.de>; Vineet Gupta <vgupta@synopsys.com>; Will Deacon <wi=
ll.deacon@arm.com>; Sasha
> Levin <alexander.levin@microsoft.com>
> Subject: Re: [PATCH AUTOSEL 4.14 72/95] devres: Align data[] to ARCH_KMAL=
LOC_MINALIGN
>=20
> On Tue, May 07, 2019 at 01:38:01AM -0400, Sasha Levin wrote:
> > From: Alexey Brodkin <alexey.brodkin@synopsys.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] https://urldefense.proofpoint.com/v2/url?u=3Dhttp-3A__lists.infrade=
ad.org_pipermail_linux-2Dsnps-
> 2Darc_2018-
> 2DJuly_004009.html&d=3DDwIBAg&c=3DDPL6_X_6JkXFx7AXWqB0tg&r=3DlqdeeSSEes0G=
FDDl656eViXO7breS55ytWkhpk5R81I&m=3DA
> YtkWKU38pzVfJMBuK0lUwxRyKT6dDfHoD3yO6OIB5k&s=3De7e2sXKcjHDQdGSrKWM0jmpSOf=
he0MFk4-nMZJe9En8&e=3D
> > [2] https://urldefense.proofpoint.com/v2/url?u=3Dhttp-3A__lists.infrade=
ad.org_pipermail_linux-2Dsnps-
> 2Darc_2018-
> 2DJuly_004036.html&d=3DDwIBAg&c=3DDPL6_X_6JkXFx7AXWqB0tg&r=3DlqdeeSSEes0G=
FDDl656eViXO7breS55ytWkhpk5R81I&m=3DA
> YtkWKU38pzVfJMBuK0lUwxRyKT6dDfHoD3yO6OIB5k&s=3DL23zrl8rf2MmReUI8rT3FQpMiZ=
U9H3Xjh9uVxJQe8dw&e=3D
> >
> > Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
> > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > Cc: Geert Uytterhoeven <geert@linux-m68k.org>
> > Cc: David Laight <David.Laight@ACULAB.COM>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Vineet Gupta <vgupta@synopsys.com>
> > Cc: Will Deacon <will.deacon@arm.com>
> > Cc: Greg KH <greg@kroah.com>
> > Cc: <stable@vger.kernel.org> # 4.8+
> > Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > Signed-off-by: Sasha Levin <alexander.levin@microsoft.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 {
>=20
> 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 :)

That very well might become the case but then we're back to the initial pro=
blem,
right? So maybe some other more future-proof solution should be implemented=
?

See initially we discussed simple explicit 8-byte alignment which won't cha=
nge
data layout for most of arches while fixing our issue on ARC but for some r=
eason
people were not happy with that proposal and that's how we ended-up with wh=
at we
discuss here now.

-Alexey