linux-kernel - [PATCH v2 1/2] mm: page_alloc: skip memoryless nodes entirely

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Message-Id: <7928768f2658cd563978f5e5bf8109be1d559320.1697687357.git.zhengqi.arch@bytedance.com>
Date:   Thu, 19 Oct 2023 15:36:26 +0800
From:   Qi Zheng <zhengqi.arch@...edance.com>
To:     akpm@...ux-foundation.org, rppt@...nel.org, david@...hat.com,
        vbabka@...e.cz, mhocko@...e.com
Cc:     willy@...radead.org, mgorman@...hsingularity.net, mingo@...nel.org,
        aneesh.kumar@...ux.ibm.com, ying.huang@...el.com,
        hannes@...xchg.org, osalvador@...e.de,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Qi Zheng <zhengqi.arch@...edance.com>
Subject: [PATCH v2 1/2] mm: page_alloc: skip memoryless nodes entirely

In find_next_best_node(), We skipped the memoryless nodes
when building the zonelists of other normal nodes (N_NORMAL),
but did not skip the memoryless node itself when building
the zonelist. This will cause it to be traversed at runtime.

For example, say we have node0 and node1, node0 is memoryless
node, then the fallback order of node0 and node1 as follows:

[    0.153005] Fallback order for Node 0: 0 1
[    0.153564] Fallback order for Node 1: 1

After this patch, we skip memoryless node0 entirely, then
the fallback order of node0 and node1 as follows:

[    0.155236] Fallback order for Node 0: 1
[    0.155806] Fallback order for Node 1: 1

So it becomes completely invisible, which will reduce runtime
overhead.

And in this way, we will not try to allocate pages from memoryless
node0, then the panic mentioned in [1] will also be fixed. Even though
this problem has been solved by dropping the NODE_MIN_SIZE constrain
in x86 [2], it would be better to fix it in core MM as well.

[1]. https://lore.kernel.org/all/20230212110305.93670-1-zhengqi.arch@bytedance.com/
[2]. https://lore.kernel.org/all/20231017062215.171670-1-rppt@kernel.org/

Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
---
 mm/page_alloc.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index ee392a324802..e978272699d3 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5052,8 +5052,11 @@ 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;
 	}
-- 
2.30.2