Re: ABI compatibility for /proc/zoneinfo

[Baoquan He <bhe@redhat.com>]

On 08/10/20 at 08:18pm, Sonny Rao 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.

Ah, I see. While the protection line can only mark the end of empty zone.
For populated zone, it obvisouly can't. One wild idea is not showing
those empty zones at all. On NUMA system with multiple memory nodes,
the nodes other than node0 won't have DMA, DMA32 zone usually, it doesn't
make any sense to list them in /proc/zoneinfo.

> 
> >
> >
> > (Note that below /proc/zoneinfo is from an old system, protection line
> > is still existing for empty zone)
> > Node 1, zone      DMA
> >   pages free     0
> >         min      0
> >         low      0
> >         high     0
> >         spanned  0
> >         present  0
> >         managed  0
> >         protection: (0, 0, 61854, 61854, 61854)
> > Node 1, zone    DMA32
> >   pages free     0
> >         min      0
> >         low      0
> >         high     0
> >         spanned  0
> >         present  0
> >         managed  0
> >         protection: (0, 0, 61854, 61854, 61854)
> > Node 1, zone   Normal
> >   per-node stats
> >       nr_inactive_anon 259
> >       nr_active_anon 11926
> > ...
> >       nr_written   0
> >       nr_kernel_misc_reclaimable 0
> >   pages free     16206452
> >   pages free     16206452
> >         min      11280
> >         low      27114
> >         high     42948
> >         spanned  16777216
> >         present  16777216
> >         managed  15834637
> >         protection: (0, 0, 0, 0, 0)
> > ...
> >
> > Node 1, zone  Movable
> >   pages free     0
> >         min      0
> >         low      0
> >         high     0
> >         spanned  0
> >         present  0
> >         managed  0
> >         protection: (0, 0, 0, 0, 0)
> > Node 1, zone   Device
> >   pages free     0
> >         min      0
> >         low      0
> >         high     0
> >         spanned  0
> >         present  0
> >         managed  0
> >         protection: (0, 0, 0, 0, 0)
> > >
> >
>