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" <[email protected]> > > > Signed-off-by: Greg Kroah-Hartman <[email protected]> > > > --- > > > 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
