Re: [PATCH v3] mm: fix panic in __alloc_pages

[Michal Hocko <mhocko@suse.com>]

On Wed 08-12-21 09:24:39, David Hildenbrand wrote:
> On 08.12.21 09:12, Michal Hocko wrote:
> > On Tue 07-12-21 19:03:28, David Hildenbrand wrote:
> >> On 07.12.21 18:17, Alexey Makhalov wrote:
> >>>
> >>>
> >>>> On Dec 7, 2021, at 9:13 AM, David Hildenbrand <david@redhat.com> wrote:
> >>>>
> >>>> On 07.12.21 18:02, Alexey Makhalov wrote:
> >>>>>
> >>>>>
> >>>>>> On Dec 7, 2021, at 8:36 AM, Michal Hocko <mhocko@suse.com> wrote:
> >>>>>>
> >>>>>> On Tue 07-12-21 17:27:29, Michal Hocko wrote:
> >>>>>> [...]
> >>>>>>> So your proposal is to drop set_node_online from the patch and add it as
> >>>>>>> a separate one which handles
> >>>>>>> 	- sysfs part (i.e. do not register a node which doesn't span a
> >>>>>>> 	  physical address space)
> >>>>>>> 	- hotplug side of (drop the pgd allocation, register node lazily
> >>>>>>> 	  when a first memblocks are registered)
> >>>>>>
> >>>>>> In other words, the first stage
> >>>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> >>>>>> index c5952749ad40..f9024ba09c53 100644
> >>>>>> --- a/mm/page_alloc.c
> >>>>>> +++ b/mm/page_alloc.c
> >>>>>> @@ -6382,7 +6382,11 @@ static void __build_all_zonelists(void *data)
> >>>>>> 	if (self && !node_online(self->node_id)) {
> >>>>>> 		build_zonelists(self);
> >>>>>> 	} else {
> >>>>>> -		for_each_online_node(nid) {
> >>>>>> +		/*
> >>>>>> +		 * All possible nodes have pgdat preallocated
> >>>>>> +		 * free_area_init
> >>>>>> +		 */
> >>>>>> +		for_each_node(nid) {
> >>>>>> 			pg_data_t *pgdat = NODE_DATA(nid);
> >>>>>>
> >>>>>> 			build_zonelists(pgdat);
> >>>>>
> >>>>> Will it blow up memory usage for the nodes which might never be onlined?
> >>>>> I prefer the idea of init on demand.
> >>>>>
> >>>>> Even now there is an existing problem.
> >>>>> In my experiments, I observed _huge_ memory consumption increase by increasing number
> >>>>> of possible numa nodes. I’m going to report it in separate mail thread.
> >>>>
> >>>> I already raised that PPC might be problematic in that regard. Which
> >>>> architecture / setup do you have in mind that can have a lot of possible
> >>>> nodes?
> >>>>
> >>> It is x86_64 VMware VM, not the regular one, but specially configured (1 vCPU per node,
> >>> with hot-plug support, 128 possible nodes)  
> >>
> >> I thought the pgdat would be smaller but I just gave it a test:
> > 
> > Yes, pgdat is quite large! Just embeded zones can eat a lot.
> > 
> >> On my system, pgdata_t is 173824 bytes. So 128 nodes would correspond to
> >> 21 MiB, which is indeed a lot. I assume it's due to "struct zonelist",
> >> which has MAX_ZONES_PER_ZONELIST == (MAX_NUMNODES * MAX_NR_ZONES) zone
> >> references ...
> > 
> > This is what pahole tells me
> > struct pglist_data {
> >         struct zone                node_zones[4] __attribute__((__aligned__(64))); /*     0  5632 */
> >         /* --- cacheline 88 boundary (5632 bytes) --- */
> >         struct zonelist            node_zonelists[1];    /*  5632    80 */
> > 	[...]
> >         /* size: 6400, cachelines: 100, members: 27 */
> >         /* sum members: 6369, holes: 5, sum holes: 31 */
> > 
> > with my particular config (which is !NUMA). I haven't really checked
> > whether there are other places which might scale with MAX_NUM_NODES or
> > something like that.
> > 
> > Anyway, is 21MB of wasted space for 128 Node machine something really
> > note worthy?
> > 
> 
> I think we'll soon might see setups (again, CXL is an example, but als
> owhen providing a dynamic amount of performance differentiated memory
> via virtio-mem) where this will most probably matter. With performance
> differentiated memory we'll see a lot more nodes getting used in
> general, and a lot more nodes eventually getting hotplugged.

There are certainly machines with many nodes. E.g. SLES kernels are
build with CONFIG_NODES_SHIFT=10 which is a lot of potential nodes.
And I have seen really large machines with many nodes but those usually
come with a lot of memory and they do not tend to have non populated
nodes AFAIR.

> If 128 nodes is realistic, I cannot tell.
> 
> We could optimize by allocating some members dynamically. For example
> we'll never need MAX_NUMNODES entries, but only the number of possible
> nodes.

Yes agreed. Scaling with MAX_NUMNODES is almost always wasteful.

-- 
Michal Hocko
SUSE Labs