On Mon 19-11-18 10:48:32, Michal Hocko wrote:
> On Sat 17-11-18 10:20:22, Wei Yang wrote:
> > Function init_currently_empty_zone() will adjust pgdat->nr_zones and set
> > it to 'zone_idx(zone) + 1' unconditionally. This is correct in the
> > normal case, while not exact in hot-plug situation.
> >
> > This function is used in two places:
> >
> > * free_area_init_core()
> > * move_pfn_range_to_zone()
> >
> > In the first case, we are sure zone index increase monotonically. While
> > in the second one, this is under users control.
> >
> > One way to reproduce this is:
> > ----------------------------
> >
> > 1. create a virtual machine with empty node1
> >
> > -m 4G,slots=32,maxmem=32G \
> > -smp 4,maxcpus=8 \
> > -numa node,nodeid=0,mem=4G,cpus=0-3 \
> > -numa node,nodeid=1,mem=0G,cpus=4-7
> >
> > 2. hot-add cpu 3-7
> >
> > cpu-add [3-7]
> >
> > 2. hot-add memory to nod1
> >
> > object_add memory-backend-ram,id=ram0,size=1G
> > device_add pc-dimm,id=dimm0,memdev=ram0,node=1
> >
> > 3. online memory with following order
> >
> > echo online_movable > memory47/state
> > echo online > memory40/state
> >
> > After this, node1 will have its nr_zones equals to (ZONE_NORMAL + 1)
> > instead of (ZONE_MOVABLE + 1).
>
> Maybe it is just me but the above was quite hard to grasp. So just to
> clarify. The underlying problem is that initialization of any existing
> empty zone will override the previous node wide setting
> (pgdat->nr_zones). The fix is to only update nr_zones when a higher zone
> is added.
>
> Fixes: f1dd2cd13c4b ("mm, memory_hotplug: do not associate hotadded memory to zones until online")
>
> I haven't checked the code prior to this rework but I suspect it was
> really the above one to change the picture.
Forgot to mention that this should probably go to stable. Having an
incorrect nr_zones might result in all sorts of problems which would be
quite hard to debug (e.g. reclaim not considering the movable zone).
I do not expect many users would suffer from this it but still this is
trivial and obviously right thing to do so backporting to the stable
tree shouldn't be harmful (last famous words).
Cc: stable # since 4.13
older tress would have to be checked explicitly.
> > Signed-off-by: Wei Yang <richard.weiyang@xxxxxxxxx>
>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
>
> Thanks!
>
> > ---
> > mm/page_alloc.c | 4 +++-
> > 1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 5b7cd20dbaef..2d3c54201255 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -5823,8 +5823,10 @@ void __meminit init_currently_empty_zone(struct zone *zone,
> > unsigned long size)
> > {
> > struct pglist_data *pgdat = zone->zone_pgdat;
> > + int zone_idx = zone_idx(zone) + 1;
> >
> > - pgdat->nr_zones = zone_idx(zone) + 1;
> > + if (zone_idx > pgdat->nr_zones)
> > + pgdat->nr_zones = zone_idx;
> >
> > zone->zone_start_pfn = zone_start_pfn;
> >
> > --
> > 2.15.1
> >
>
> --
> Michal Hocko
> SUSE Labs
--
Michal Hocko
SUSE Labs
