Re: [PATCH v2 2/9] mm/vmstat: show start_pfn when zone spans pages

[Doug Berger]

On 9/29/2022 1:15 AM, David Hildenbrand wrote:
> On 29.09.22 00:32, Doug Berger wrote:
> > A zone that overlaps with another zone may span a range of pages
> > that are not present. In this case, displaying the start_pfn of
> > the zone allows the zone page range to be identified.
>
> I don't understand the intention here.
>
> "/* If unpopulated, no other information is useful */"
>
> Why would the start pfn be of any use here?
>
> What is the user visible impact without that change?
Yes, this is very subtle. I only caught it while testing some 
pathological cases.

If you take the example system:
The 7278 device has four ARMv8 CPU cores in an SMP cluster and two 
memory controllers (MEMCs). Each MEMC is capable of controlling up to 
8GB of DRAM. An example 7278 system might have 1GB on each controller, 
so an arm64 kernel might see 1GB on MEMC0 at 0x40000000-0x7FFFFFFF and 
1GB on MEMC1 at 0x300000000-0x33FFFFFFF.

Placing a DMB on MEMC0 with 'movablecore=256M@0x70000000' will lead to 
the ZONE_MOVABLE zone spanning from 0x70000000-0x33fffffff and the 
ZONE_NORMAL zone spanning from 0x300000000-0x33fffffff.

If instead you specified 'movablecore=256M@0x70000000,512M' you would 
get the same ZONE_MOVABLE span, but the ZONE_NORMAL would now span 
0x300000000-0x32fffffff. The requested 512M of movablecore would be 
divided into a 256MB DMB at 0x70000000 and a 256MB "classic" movable 
zone start would be displayed in the bootlog as:
[    0.000000] Movable zone start for each node
[    0.000000]   Node 0: 0x000000330000000

Finally, if you specified the pathological 
'movablecore=256M@0x70000000,1G@12G' you would still have the same 
ZONE_MOVABLE span, and the ZONE_NORMAL span would go back to 
0x300000000-0x33fffffff. However, because the second DMB (1G@12G) 
completely overlaps the ZONE_NORMAL there would be no pages present in 
ZONE_NORMAL and /proc/zoneinfo would report ZONE_NORMAL 'spanned 
262144', but not where those pages are. This commit adds the 'start_pfn' 
back to the /proc/zoneinfo for ZONE_NORMAL so the span has context.

Regards,
    Doug

> > Signed-off-by: Doug Berger <opendmb@xxxxxxxxx>
> > ---
> >   mm/vmstat.c | 5 +++++
> >   1 file changed, 5 insertions(+)
> >
> > diff --git a/mm/vmstat.c b/mm/vmstat.c
> > index 90af9a8572f5..e2f19f2b7615 100644
> > --- a/mm/vmstat.c
> > +++ b/mm/vmstat.c
> > @@ -1717,6 +1717,11 @@ static void zoneinfo_show_print(struct seq_file 
> > *m, pg_data_t *pgdat,
> >       /* If unpopulated, no other information is useful */
> >       if (!populated_zone(zone)) {
> > +        /* Show start_pfn for empty overlapped zones */
> > +        if (zone->spanned_pages)
> > +            seq_printf(m,
> > +                   "\n  start_pfn:           %lu",
> > +                   zone->zone_start_pfn);
> >           seq_putc(m, '\n');
> >           return;
> >       }