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

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Date:   Tue, 7 Dec 2021 16:56:13 +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 16:34:30, David Hildenbrand wrote:
> On 07.12.21 16:29, Michal Hocko wrote:
> > On Tue 07-12-21 16:09:39, David Hildenbrand wrote:
> >> On 07.12.21 14:23, Michal Hocko wrote:
> >>> On Tue 07-12-21 13:28:31, David Hildenbrand wrote:
> >>> [...]
> >>>> But maybe I am missing something important regarding online vs. offline
> >>>> nodes that your patch changes?
> >>>
> >>> I am relying on alloc_node_data setting the node online. But if we are
> >>> to change the call to arch_alloc_node_data then the patch needs to be
> >>> more involved. Here is what I have right now. If this happens to be the
> >>> right way then there is some additional work to sync up with the hotplug
> >>> code.
> >>>
> >>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> >>> index c5952749ad40..a296e934ad2f 100644
> >>> --- a/mm/page_alloc.c
> >>> +++ b/mm/page_alloc.c
> >>> @@ -8032,8 +8032,23 @@ void __init free_area_init(unsigned long *max_zone_pfn)
> >>>  	/* Initialise every node */
> >>>  	mminit_verify_pageflags_layout();
> >>>  	setup_nr_node_ids();
> >>> -	for_each_online_node(nid) {
> >>> -		pg_data_t *pgdat = NODE_DATA(nid);
> >>> +	for_each_node(nid) {
> >>> +		pg_data_t *pgdat;
> >>> +
> >>> +		if (!node_online(nid)) {
> >>> +			pr_warn("Node %d uninitialized by the platform. Please report with memory map.\n", nid);
> >>> +			pgdat = arch_alloc_nodedata(nid);
> >>> +			pgdat->per_cpu_nodestats = alloc_percpu(struct per_cpu_nodestat);
> >>> +			arch_refresh_nodedata(nid, pgdat);
> >>> +			node_set_online(nid);
> >>
> >> Setting all possible nodes online might result in quite some QE noice,
> >> because all these nodes will then be visible in the sysfs and
> >> try_offline_nodes() is essentially for the trash.
> > 
> > I am not sure I follow. I believe sysfs will not get populate because I
> > do not call register_one_node.
> 
> arch/x86/kernel/topology.c:topology_init()
> 
> for_each_online_node(i)
> 	register_one_node(i);

Right you are.
 
> > You are right that try_offline_nodes will be reduce which is good imho.
> > More changes will be possible (hopefully to drop some ugly code) on top
> > of this change (or any other that achieves that there are no NULL pgdat
> > for possible nodes).
> > 
> 
> No to exposing actually offline nodes to user space via sysfs.

Why is that a problem with the sysfs for non-populated nodes?

-- 
Michal Hocko
SUSE Labs