linux-kernel - Re: [PATCH 1/2] device.h: pack struct dev_links

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Message-ID: <20190227094021.GL4747@localhost>
Date:   Wed, 27 Feb 2019 10:40:21 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Johan Hovold <johan@...nel.org>, linux-kernel@...r.kernel.org,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH 1/2] device.h: pack struct dev_links_info

On Wed, Feb 27, 2019 at 10:31:04AM +0100, Greg Kroah-Hartman wrote:
> On Wed, Feb 27, 2019 at 10:23:18AM +0100, Johan Hovold wrote:
> > On Tue, Feb 26, 2019 at 03:41:07PM +0100, Greg Kroah-Hartman wrote:
> > > The dev_links_info structure has 4 bytes of padding at the end of it
> > > when embedded in struct device (which is the only place it lives).  To
> > > help reduce the size of struct device pack this structure so we can take
> > > advantage of the hole with later structure reorganizations.
> > > 
> > > Cc: "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
> > > Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > > ---
> > >  include/linux/device.h | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/include/linux/device.h b/include/linux/device.h
> > > index 6cb4640b6160..b63165276a09 100644
> > > --- a/include/linux/device.h
> > > +++ b/include/linux/device.h
> > > @@ -884,7 +884,7 @@ struct dev_links_info {
> > >  	struct list_head suppliers;
> > >  	struct list_head consumers;
> > >  	enum dl_dev_state status;
> > > -};
> > > +} __packed;
> > 
> > This seems like a bad idea. You're changing the alignment of these
> > fields to one byte, something which may cause the compiler to generate
> > less efficient code to deal with unaligned accesses (even if they happen
> > to currently be naturally aligned in struct device).
> 
> No, all this changes is the trailing "space" is gone.  The alignment of
> the fields did not change at all as they are all naturally aligned
> (list_head is just 2 pointers).

Yes, currently and in struct device, but given a pointer to a struct
dev_links_info the compiler must assume it is unaligned and act
accordingly for example.

> So this allows us to save 4 bytes in struct device by putting something in that
> trailing "hole" that can be aligned with it better (i.e. an integer or
> something else).

I understand that, but I don't think it is worth to start using packed
liked this for internal structures as it may have subtle and unintended
consequences.

Johan