On Tue, 1 Oct 2019 at 22:57, Johannes Berg <johannes@xxxxxxxxxxxxxxxx> wrote:
>
> Hi,
>
Hi!
> (jumping out now, forgive me for being so brief)
>
> > If I understand correctly, you said about the alignment of
> > "lower_level" and "upper_level".
> > I thought this place is a fine position for variables as regards the
> > alignment and I didn't try to put each variable in different places.
> >
> > If I misunderstood your mention, please let me know.
>
> Not sure what you mean, alignment doesn't matter for them (they're u8).
>
> I was thinking of the packing for the overall struct, we have:
>
> unsigned int max_mtu;
> unsigned short type;
> unsigned short hard_header_len;
> unsigned char min_header_len;
>
> + unsigned char upper_level, lower_level;
>
> unsigned short needed_headroom;
> unsigned short needed_tailroom;
>
>
> Previously, there was a one byte hole at that spot due to a single
> "unsigned char" (after something aligned at least 4 bytes) followed by
> "unsigned short" - now you push that out a bit.
>
> If you place the variables a bit lower, below "name_assign_type", you
> probably fill a hole instead.
>
> Check out the 'pahole' tool.
>
Thank you for the detailed explanation.
I tested the pahole and found holes.
$ pahole ./vmlinux.o -C net_device
unsigned char addr_assign_type; /* 598 1 */
unsigned char addr_len; /* 599 1 */
short unsigned int neigh_priv_len; /* 600 2 */
short unsigned int dev_id; /* 602 2 */
short unsigned int dev_port; /* 604 2 */
/* XXX 2 bytes hole, try to pack */
I will place the variables here.
> johannes
>
Thank you so much!
Taehee
