Re: [PATCH] mm: page_alloc: don't allocate page from memoryless nodes

[Qi Zheng]

On 2023/2/14 19:44, Mike Rapoport wrote:
> (added x86 folks)
>
> On Tue, Feb 14, 2023 at 12:29:42PM +0100, David Hildenbrand wrote:
> > On 14.02.23 12:26, Qi Zheng wrote:
> > > On 2023/2/14 19:22, David Hildenbrand wrote:
> > > > TBH, this is the first time I hear of NODE_MIN_SIZE and it seems to be a
> > > > pretty x86 specific thing.
> > > >
> > > > Are we sure we want to get NODE_MIN_SIZE involved?
> > >
> > > Maybe add an arch_xxx() to handle it?
> >
> > I still haven't figured out what we want to achieve with NODE_MIN_SIZE at
> > all. It smells like an arch-specific hack looking at
> >
> > "Don't confuse VM with a node that doesn't have the minimum amount of
> > memory"
> >
> > Why shouldn't mm-core deal with that?
>
> Well, a node with <4M RAM is not very useful and bears all the overhead of
> an extra live node.
>
> But, hey, why won't we just drop that '< NODE_MIN_SIZE' and let people with
> weird HW configurations just live with this?

Just to sum up, whether we deal with '< NODE_MIN_SIZE' or not, IIUC, the
following two should be modified:

1) we should skip memoryless nodes completely in find_next_best_node():

@@ -6382,8 +6378,11 @@ int find_next_best_node(int node, nodemask_t
*used_node_mask)
         int min_val = INT_MAX;
         int best_node = NUMA_NO_NODE;

-       /* Use the local node if we haven't already */
-       if (!node_isset(node, *used_node_mask)) {
+       /*
+        * Use the local node if we haven't already. But for memoryless
local
+        * node, we should skip it and fallback to other nodes.
+        */
+       if (!node_isset(node, *used_node_mask) && node_state(node,
N_MEMORY)) {
                 node_set(node, *used_node_mask);
                 return node;
         }

This also fixes the bug mentioned in commit message.

2) we should call node_states_clear_node() before build_all_zonelists()
in offline_pages():

@@ -1931,12 +1931,12 @@ int __ref offline_pages(unsigned long start_pfn, 
unsigned long nr_pages,
        /* reinitialise watermarks and update pcp limits */
        init_per_zone_wmark_min();

+       node_states_clear_node(node, &arg);
        if (!populated_zone(zone)) {
                zone_pcp_reset(zone);
                build_all_zonelists(NULL);
        }

-       node_states_clear_node(node, &arg);
        if (arg.status_change_nid >= 0) {
                kcompactd_stop(node);
                kswapd_stop(node);

Otherwise, the node whose N_MEMORY state is about to be cleared will 
still be established in the fallback list of other nodes.

Right?

Thanks,
Qi

>   
> > I'd appreciate an explanation of the bigger picture, what the issue is and
> > what the approach to solve it is (including memory onlining/offlining).
> >
> > --
> > Thanks,
> >
> > David / dhildenb

--
Thanks,
Qi