Re: [PATCH v6 01/13] mm/demotion: Add support for explicit memory tiers

From: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
To: Ying Huang <ying.huang@intel.com>,
	linux-mm@kvack.org, akpm@linux-foundation.org
Cc: Wei Xu <weixugc@google.com>, Greg Thelen <gthelen@google.com>,
	Yang Shi <shy828301@gmail.com>,
	Davidlohr Bueso <dave@stgolabs.net>,
	Tim C Chen <tim.c.chen@intel.com>,
	Brice Goglin <brice.goglin@gmail.com>,
	Michal Hocko <mhocko@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Hesham Almatary <hesham.almatary@huawei.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Jonathan Cameron <Jonathan.Cameron@huawei.com>,
	Alistair Popple <apopple@nvidia.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Feng Tang <feng.tang@intel.com>,
	Jagdish Gediya <jvgediya@linux.ibm.com>,
	Baolin Wang <baolin.wang@linux.alibaba.com>,
	David Rientjes <rientjes@google.com>,
	Johannes Weiner <hannes@cmpxchg.org>
Subject: Re: [PATCH v6 01/13] mm/demotion: Add support for explicit memory tiers
Date: Tue, 14 Jun 2022 13:50:29 +0530	[thread overview]
Message-ID: <87a6afmzaq.fsf@linux.ibm.com> (raw)
In-Reply-To: <a88bd25fc77252dee4f895f3a9b2c1f6ebb5169e.camel@intel.com>

Ying Huang <ying.huang@intel.com> writes:

....
> 
>> All those functions are called with memory_tier_lock_held. Infact all 
>> list operations requires that lock held. What details do you suggest we 
>> document? I can add extra comment to the mutex itself? Adding locking 
>> details to all the functions will be duplicating the same details at 
>> multiple places?
>
> memory_tier_lock isn't held to call register_memory_tier() in this
> patch.  That will cause confusion.

will this help to explain this better
modified   mm/memory-tiers.c
@@ -151,6 +151,11 @@ static void insert_memory_tier(struct memory_tier *memtier)
 	struct list_head *ent;
 	struct memory_tier *tmp_memtier;
 
+	if (IS_ENABLED(CONFIG_DEBUG_VM) && !mutex_is_locked(&memory_tier_lock)) {
+		WARN_ON_ONCE(1);
+		return;
+	}
+
 	list_for_each(ent, &memory_tiers) {
 		tmp_memtier = list_entry(ent, struct memory_tier, list);
 		if (tmp_memtier->rank < memtier->rank) {
@@ -811,8 +816,12 @@ static int __init memory_tier_init(void)
 
 	/*
 	 * Register only default memory tier to hide all empty
-	 * memory tier from sysfs.
+	 * memory tier from sysfs. Since this is early during
+	 * boot, we could avoid holding memtory_tier_lock. But
+	 * keep it simple by holding locks. We can add lock
+	 * held debug checks in other functions.
 	 */
+	mutex_lock(&memory_tier_lock);
 	memtier = register_memory_tier(DEFAULT_MEMORY_TIER,
 				       get_rank_from_tier(DEFAULT_MEMORY_TIER));
 
@@ -828,6 +837,7 @@ static int __init memory_tier_init(void)
 		NODE_DATA(node)->memtier = memtier;
 		node_set(node, memtier->nodelist);
 	}
+	mutex_unlock(&memory_tier_lock);
 	migrate_on_reclaim_init();
 
 	return 0;

-aneesh
