Re: ABI compatibility for /proc/zoneinfo

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 10 Aug 2020 20:18:11 -0700 Sonny Rao <sonnyrao@chromium.org> wrote:

> On Mon, Aug 10, 2020 at 7:37 PM Baoquan He <bhe@redhat.com> wrote:
> >
> > On 08/10/20 at 03:46pm, Yu Zhao wrote:
> > > On Mon, Aug 10, 2020 at 02:24:03PM -0700, Sonny Rao wrote:
> > > > We (Chrome OS) noticed recently one of our tests started failing on
> > > > upstream kernels while parsing /proc/zoneinfo
> > > > I think this patch is the cause:
> > > >
> > > > 26e7deadaae17 mm/vmstat.c: do not show lowmem reserve protection
> > > > information of empty zone
> > > >
> > > > Maybe our parser was being overly strict by looking for the protection
> > > > line, and it's not hard to fix but raised the question of whether there's
> > > > any ABI compatibility guarantees about these files?
> > >
> > > According to Documentation/admin-guide/sysctl/vm.rst, "Each zone has
> > > an array of protection pages". I'm not sure if this is the guarantee,
> > > but the doc should reflect the actual format.
> >
> > The current code will list all zones in one memory node, even though
> > that node only has one existing zone. E.g in below node 1,
> > it only has NORMAL zone, but we will list zone DMA, DMA32, MOVABLE,
> > DEVICE which are all empty zone, namely doesn't exist. So, each zone
> > has an array of protection pages, it should not include the nonexistent
> > zone. I thought nobody would check the protection line of an empty zone,
> > seems I was wrong.
> 
> 
> This particular parser was written as a state machine and that line
> was a convenient thing to look for to mark the end of each zone.
> AFAICT, there's no explicit documentation on the layout of that file
> though.

This only affects 5.8 and later, yes?

26e7deadaae17 didn't gain us much at all and we shouldn't break
userspace unless it's super important.  So I think it's best to revert,
and to backport the revert to 5.8.x.

Could someone (Baoquan He?) please send a patch with a suitable
changelog, cc:stable, reported-by:, etc?

Thanks.