linux-kernel - Re: [PATCH v3] mm: fix panic in __alloc

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Date:   Wed, 8 Dec 2021 09:12:22 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Alexey Makhalov <amakhalov@...are.com>,
        Dennis Zhou <dennis@...nel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH v3] mm: fix panic in __alloc_pages

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@...hat.com> wrote:
> >>
> >> On 07.12.21 18:02, Alexey Makhalov wrote:
> >>>
> >>>
> >>>> On Dec 7, 2021, at 8:36 AM, Michal Hocko <mhocko@...e.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?
-- 
Michal Hocko
SUSE Labs