Re: [RFC 1/3] /dev/low_mem_notify

[Arnd Bergmann <arnd@arndb.de>]

On Wednesday 18 January 2012, Pekka Enberg wrote:
> >> +struct vmnotify_event {
> >> +     /* Size of the struct for ABI extensibility. */
> >> +     __u32                   size;
> >> +
> >> +     __u64                   nr_avail_pages;
> >> +
> >> +     __u64                   nr_swap_pages;
> >> +
> >> +     __u64                   nr_free_pages;
> >> +};
> >
> > Two fields here most likely session-constant, (nr_avail_pages and
> > nr_swap_pages), seems not much sense to report them in every event.  If we
> > have memory/swap hotplug user-space can use sysinfo() call.
> 
> I actually changed the ABI to look like this:
> 
> struct vmnotify_event {
>         /*
>          * Size of the struct for ABI extensibility.
>          */
>         __u32                   size;
> 
>         __u64                   attrs;
> 
>         __u64                   attr_values[];
> };
> 
> So userspace can decide which fields to include in notifications.

Please make the first member a __u64 instead of a __u32. This will
avoid incompatibility between 32 and 64 bit processes, which have
different alignment rules on x86: x86-32 would implicitly pack the
struct while x86-64 would add padding with your layout.

	Arnd