Re: [PATCH AUTOSEL 4.14 72/95] devres: Align data[] to ARCH_KMALLOC_MINALIGN

[Greg Kroah-Hartman]

On Tue, May 07, 2019 at 07:04:13AM +0000, Alexey Brodkin wrote:
> Hi Greg,
> 
> > -----Original Message-----
> > From: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > Sent: Tuesday, May 7, 2019 8:52 AM
> > To: Sasha Levin <sashal@xxxxxxxxxx>
> > Cc: linux-kernel@xxxxxxxxxxxxxxx; stable@xxxxxxxxxxxxxxx; Alexey Brodkin <abrodkin@xxxxxxxxxxxx>;
> > Alexey Brodkin <abrodkin@xxxxxxxxxxxx>; Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>; David Laight
> > <David.Laight@xxxxxxxxxx>; Peter Zijlstra <peterz@xxxxxxxxxxxxx>; Thomas Gleixner
> > <tglx@xxxxxxxxxxxxx>; Vineet Gupta <vgupta@xxxxxxxxxxxx>; Will Deacon <will.deacon@xxxxxxx>; Sasha
> > Levin <alexander.levin@xxxxxxxxxxxxx>
> > 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@xxxxxxxxxxxx>
> > >
> > > [ 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=http-3A__lists.infradead.org_pipermail_linux-2Dsnps-
> > 2Darc_2018-
> > 2DJuly_004009.html&d=DwIBAg&c=DPL6_X_6JkXFx7AXWqB0tg&r=lqdeeSSEes0GFDDl656eViXO7breS55ytWkhpk5R81I&m=A
> > YtkWKU38pzVfJMBuK0lUwxRyKT6dDfHoD3yO6OIB5k&s=e7e2sXKcjHDQdGSrKWM0jmpSOfhe0MFk4-nMZJe9En8&e=
> > > [2] https://urldefense.proofpoint.com/v2/url?u=http-3A__lists.infradead.org_pipermail_linux-2Dsnps-
> > 2Darc_2018-
> > 2DJuly_004036.html&d=DwIBAg&c=DPL6_X_6JkXFx7AXWqB0tg&r=lqdeeSSEes0GFDDl656eViXO7breS55ytWkhpk5R81I&m=A
> > YtkWKU38pzVfJMBuK0lUwxRyKT6dDfHoD3yO6OIB5k&s=L23zrl8rf2MmReUI8rT3FQpMiZU9H3Xjh9uVxJQe8dw&e=
> > >
> > > Signed-off-by: Alexey Brodkin <abrodkin@xxxxxxxxxxxx>
> > > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > > Cc: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
> > > Cc: David Laight <David.Laight@xxxxxxxxxx>
> > > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > > Cc: Vineet Gupta <vgupta@xxxxxxxxxxxx>
> > > Cc: Will Deacon <will.deacon@xxxxxxx>
> > > Cc: Greg KH <greg@xxxxxxxxx>
> > > Cc: <stable@xxxxxxxxxxxxxxx> # 4.8+
> > > Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > > Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
> > > ---
> > >  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 :)
> 
> That very well might become the case but then we're back to the initial problem,
> right? So maybe some other more future-proof solution should be implemented?

Possibly yes.

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

I'm not disagreeing that this is a valid solution for you, I wasn't part
of the original discussion, sorry.  Just that this probably isn't
something that should be backported to older kernels at this point in
time.

thanks,

greg k-h