linux-kernel - Re: [RFC 1/3] /dev/low_mem

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Message-Id: <201201241622.36222.arnd@arndb.de>
Date:	Tue, 24 Jan 2012 16:22:36 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	Pekka Enberg <penberg@...nel.org>
Cc:	leonid.moiseichuk@...ia.com, riel@...hat.com, minchan@...nel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	kamezawa.hiroyu@...fujitsu.com, mel@....ul.ie, rientjes@...gle.com,
	kosaki.motohiro@...il.com, hannes@...xchg.org, mtosatti@...hat.com,
	akpm@...ux-foundation.org, rhod@...hat.com,
	kosaki.motohiro@...fujitsu.com
Subject: Re: [RFC 1/3] /dev/low_mem_notify

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
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