Message-ID: <632e6be8-f1e2-b57f-a70c-f3aec3adabd1@bytedance.com>
Date:   Tue, 14 Feb 2023 20:33:00 +0800
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Subject: Re: [PATCH] mm: page_alloc: don't allocate page from memoryless nodes
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To:     Mike Rapoport <rppt@kernel.org>,
        David Hildenbrand <david@redhat.com>
Cc:     Qi Zheng <zhengqi.arch@bytedance.com>,
        Vlastimil Babka <vbabka@suse.cz>, akpm@linux-foundation.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Teng Hu <huteng.ht@bytedance.com>,
        Matthew Wilcox <willy@infradead.org>,
        Mel Gorman <mgorman@techsingularity.net>,
        Oscar Salvador <osalvador@suse.de>,
        Muchun Song <muchun.song@linux.dev>
References: <20230212110305.93670-1-zhengqi.arch@bytedance.com>
 <2484666e-e78e-549d-e075-b2c39d460d71@suse.cz>
 <85af4ada-96c8-1f99-90fa-9b6d63d0016e@bytedance.com>
 <c6908bd0-5f76-47eb-2b77-ce8f7bf9fbff@suse.cz>
 <eefc40e5-a14b-22c5-3480-6786afa1c8f4@redhat.com>
 <Y+tXrK/g1Nrd/q1h@kernel.org>
 <67240e55-af49-f20a-2b4b-b7d574cd910d@gmail.com>
 <22f0e262-982e-ea80-e52a-a3c924b31d58@redhat.com>
 <dbfbd982-27f3-0d72-49e0-d3dd5fe636a8@bytedance.com>
 <4386151c-0328-d207-9a71-933ef61817f9@redhat.com>
 <Y+t0Bhu7BCzH2Dp4@kernel.org>
From:   Qi Zheng <zhengqi.arch@bytedance.com>
In-Reply-To: <Y+t0Bhu7BCzH2Dp4@kernel.org>
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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