Re: [PATCH 3/3] mm: oom: show unreclaimable slab info when unreclaimable slabs > user memory

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On 10/4/17 7:27 AM, Michal Hocko wrote:
On Wed 04-10-17 02:06:17, Yang Shi wrote:
+static bool is_dump_unreclaim_slabs(void)
+{
+	unsigned long nr_lru;
+
+	nr_lru = global_node_page_state(NR_ACTIVE_ANON) +
+		 global_node_page_state(NR_INACTIVE_ANON) +
+		 global_node_page_state(NR_ACTIVE_FILE) +
+		 global_node_page_state(NR_INACTIVE_FILE) +
+		 global_node_page_state(NR_ISOLATED_ANON) +
+		 global_node_page_state(NR_ISOLATED_FILE) +
+		 global_node_page_state(NR_UNEVICTABLE);
+
+	return (global_node_page_state(NR_SLAB_UNRECLAIMABLE) > nr_lru);
+}

I am sorry I haven't pointed this earlier (I was following only half
way) but this should really be memcg aware. You are checking only global
counters. I do not think it is an absolute must to provide per-memcg
data but you should at least check !is_memcg_oom(oc).

OK, sure.


[...]
+void dump_unreclaimable_slab(void)
+{
+	struct kmem_cache *s, *s2;
+	struct slabinfo sinfo;
+
+	pr_info("Unreclaimable slab info:\n");
+	pr_info("Name                      Used          Total\n");
+
+	/*
+	 * Here acquiring slab_mutex is risky since we don't prefer to get
+	 * sleep in oom path. But, without mutex hold, it may introduce a
+	 * risk of crash.
+	 * Use mutex_trylock to protect the list traverse, dump nothing
+	 * without acquiring the mutex.
+	 */
+	if (!mutex_trylock(&slab_mutex))
+		return;

I would move the trylock up so that we do not get empty and confusing
Unreclaimable slab info: and add a note that we are not dumping anything
due to lock contention
	pr_warn("excessive unreclaimable slab memory but cannot dump stats to give you more details\n");

Thanks for pointing this. Will fix in new version.

Yang


Other than that this looks sensible to me.

+	list_for_each_entry_safe(s, s2, &slab_caches, list) {
+		if (!is_root_cache(s) || (s->flags & SLAB_RECLAIM_ACCOUNT))
+			continue;
+
+		memset(&sinfo, 0, sizeof(sinfo));
+		get_slabinfo(s, &sinfo);
+
+		if (sinfo.num_objs > 0)
+			pr_info("%-17s %10luKB %10luKB\n", cache_name(s),
+				(sinfo.active_objs * s->size) / 1024,
+				(sinfo.num_objs * s->size) / 1024);
+	}
+	mutex_unlock(&slab_mutex);
+}
+
  #if defined(CONFIG_MEMCG) && !defined(CONFIG_SLOB)
  void *memcg_slab_start(struct seq_file *m, loff_t *pos)
  {
--
1.8.3.1


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