Date:   Thu, 1 Sep 2022 14:35:42 +0800
From:   Muchun Song <muchun.song@...ux.dev>
To:     David Hildenbrand <david@...hat.com>
Cc:     Muchun Song <songmuchun@...edance.com>,
        Greg KH <gregkh@...uxfoundation.org>, rafael@...nel.org,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>, andi@...stfloor.org,
        linux-kernel@...r.kernel.org, Linux MM <linux-mm@...ck.org>,
        David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH RESEND] mm: hugetlb: simplify per-node sysfs creation and
 removal



> On Aug 24, 2022, at 11:23, Muchun Song <muchun.song@...ux.dev> wrote:
> 
> 
> 
>> On Aug 23, 2022, at 18:21, David Hildenbrand <david@...hat.com> wrote:
>> 
>> On 19.08.22 10:00, Muchun Song wrote:
>>> The following commit offload per-node sysfs creation and removal to a kworker and
>>> did not say why it is needed.  And it also said "I don't know that this is
>>> absolutely required".  It seems like the author was not sure as well.  Since it
>>> only complicates the code, this patch will revert the changes to simplify the code.
>>> 
>>> 39da08cb074c ("hugetlb: offload per node attribute registrations")
>>> 
>>> We could use memory hotplug notifier to do per-node sysfs creation and removal
>>> instead of inserting those operations to node registration and unregistration.
>>> Then, it can reduce the code coupling between node.c and hugetlb.c.  Also, it can
>>> simplify the code.
>>> 
>>> Signed-off-by: Muchun Song <songmuchun@...edance.com>
>> 
>> 
>> [...]
>> 
>>> @@ -683,7 +626,6 @@ static int register_node(struct node *node, int num)
>>> void unregister_node(struct node *node)
>>> {
>>> 	compaction_unregister_node(node);
>>> -	hugetlb_unregister_node(node);		/* no-op, if memoryless node */
>>> 	node_remove_accesses(node);
>>> 	node_remove_caches(node);
>>> 	device_unregister(&node->dev);
>>> @@ -905,74 +847,8 @@ void register_memory_blocks_under_node(int nid, unsigned long start_pfn,
>>> 			   (void *)&nid, func);
>>> 	return;
>>> }
>> 
>> [...]
>> 
>>> 	/*
>>> 	 * Create all node devices, which will properly link the node
>>> 	 * to applicable memory block devices and already created cpu devices.
>>> diff --git a/include/linux/node.h b/include/linux/node.h
>>> index 40d641a8bfb0..ea817b507f54 100644
>>> --- a/include/linux/node.h
>>> +++ b/include/linux/node.h
>>> @@ -2,15 +2,15 @@
>>> /*
>>> * include/linux/node.h - generic node definition
>>> *
>>> - * This is mainly for topological representation. We define the 
>>> - * basic 'struct node' here, which can be embedded in per-arch 
>>> + * This is mainly for topological representation. We define the
>>> + * basic 'struct node' here, which can be embedded in per-arch
>>> * definitions of processors.
>>> *
>>> * Basic handling of the devices is done in drivers/base/node.c
>>> - * and system devices are handled in drivers/base/sys.c. 
>>> + * and system devices are handled in drivers/base/sys.c.
>>> *
>>> * Nodes are exported via driverfs in the class/node/devices/
>>> - * directory. 
>>> + * directory.
>> 
>> Unrelated changes.
> 
> Yep, a minor cleanup BTW.
> 
>> 
>>> */
>>> #ifndef _LINUX_NODE_H_
>>> #define _LINUX_NODE_H_
>>> @@ -18,7 +18,6 @@
>>> #include <linux/device.h>
>>> #include <linux/cpumask.h>
>>> #include <linux/list.h>
>>> -#include <linux/workqueue.h>
>>> 
>>> /**
>>> * struct node_hmem_attrs - heterogeneous memory performance attributes
>>> @@ -84,10 +83,6 @@ static inline void node_set_perf_attrs(unsigned int nid,
>>> struct node {
>>> 	struct device	dev;
>>> 	struct list_head access_list;
>>> -
>>> -#if defined(CONFIG_MEMORY_HOTPLUG) && defined(CONFIG_HUGETLBFS)
>>> -	struct work_struct	node_work;
>>> -#endif
>>> #ifdef CONFIG_HMEM_REPORTING
>>> 	struct list_head cache_attrs;
>>> 	struct device *cache_dev;
>>> @@ -96,7 +91,6 @@ struct node {
>>> 
>>> struct memory_block;
>>> extern struct node *node_devices[];
>>> -typedef  void (*node_registration_func_t)(struct node *);
>>> 
>>> #if defined(CONFIG_MEMORY_HOTPLUG) && defined(CONFIG_NUMA)
>>> void register_memory_blocks_under_node(int nid, unsigned long start_pfn,
>>> @@ -144,11 +138,6 @@ extern void unregister_memory_block_under_nodes(struct memory_block *mem_blk);
>>> extern int register_memory_node_under_compute_node(unsigned int mem_nid,
>>> 						   unsigned int cpu_nid,
>>> 						   unsigned access);
>>> -
>>> -#ifdef CONFIG_HUGETLBFS
>>> -extern void register_hugetlbfs_with_node(node_registration_func_t doregister,
>>> -					 node_registration_func_t unregister);
>>> -#endif
>>> #else
>>> static inline void node_dev_init(void)
>>> {
>>> @@ -176,11 +165,6 @@ static inline int unregister_cpu_under_node(unsigned int cpu, unsigned int nid)
>>> static inline void unregister_memory_block_under_nodes(struct memory_block *mem_blk)
>>> {
>>> }
>>> -
>>> -static inline void register_hugetlbfs_with_node(node_registration_func_t reg,
>>> -						node_registration_func_t unreg)
>>> -{
>>> -}
>>> #endif
>>> 
>>> #define to_node(device) container_of(device, struct node, dev)
>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>> index 536a52c29035..9a72499486c1 100644
>>> --- a/mm/hugetlb.c
>>> +++ b/mm/hugetlb.c
>>> @@ -33,6 +33,7 @@
>>> #include <linux/migrate.h>
>>> #include <linux/nospec.h>
>>> #include <linux/delayacct.h>
>>> +#include <linux/memory.h>
>>> 
>>> #include <asm/page.h>
>>> #include <asm/pgalloc.h>
>>> @@ -3967,19 +3968,19 @@ static void hugetlb_unregister_node(struct node *node)
>>> * Register hstate attributes for a single node device.
>>> * No-op if attributes already registered.
>>> */
>>> -static void hugetlb_register_node(struct node *node)
>>> +static int hugetlb_register_node(struct node *node)
>>> {
>>> 	struct hstate *h;
>>> 	struct node_hstate *nhs = &node_hstates[node->dev.id];
>>> 	int err;
>>> 
>>> 	if (nhs->hugepages_kobj)
>>> -		return;		/* already allocated */
>>> +		return 0;		/* already allocated */
>>> 
>>> 	nhs->hugepages_kobj = kobject_create_and_add("hugepages",
>>> 							&node->dev.kobj);
>>> 	if (!nhs->hugepages_kobj)
>>> -		return;
>>> +		return -ENOMEM;
>>> 
>>> 	for_each_hstate(h) {
>>> 		err = hugetlb_sysfs_add_hstate(h, nhs->hugepages_kobj,
>>> @@ -3989,9 +3990,28 @@ static void hugetlb_register_node(struct node *node)
>>> 			pr_err("HugeTLB: Unable to add hstate %s for node %d\n",
>>> 				h->name, node->dev.id);
>>> 			hugetlb_unregister_node(node);
>>> -			break;
>>> +			return -ENOMEM;
>>> 		}
>>> 	}
>>> +	return 0;
>>> +}
>>> +
>>> +static int __meminit hugetlb_memory_callback(struct notifier_block *self,
>>> +					     unsigned long action, void *arg)
>>> +{
>>> +	int ret = 0;
>>> +	struct memory_notify *mnb = arg;
>>> +	int nid = mnb->status_change_nid;
>>> +
>>> +	if (nid == NUMA_NO_NODE)
>>> +		return NOTIFY_DONE;
>>> +
>>> +	if (action == MEM_GOING_ONLINE)
>>> +		ret = hugetlb_register_node(node_devices[nid]);
>>> +	else if (action == MEM_CANCEL_ONLINE || action == MEM_OFFLINE)
>>> +		hugetlb_unregister_node(node_devices[nid]);
>>> +
>>> +	return notifier_from_errno(ret);
>>> }
>>> 
>>> /*
>>> @@ -4003,18 +4023,11 @@ static void __init hugetlb_register_all_nodes(void)
>>> {
>>> 	int nid;
>>> 
>>> -	for_each_node_state(nid, N_MEMORY) {
>>> -		struct node *node = node_devices[nid];
>>> -		if (node->dev.id == nid)
>>> -			hugetlb_register_node(node);
>>> -	}
>>> -
>>> -	/*
>>> -	 * Let the node device driver know we're here so it can
>>> -	 * [un]register hstate attributes on node hotplug.
>>> -	 */
>>> -	register_hugetlbfs_with_node(hugetlb_register_node,
>>> -				     hugetlb_unregister_node);
>>> +	get_online_mems();
>>> +	hotplug_memory_notifier(hugetlb_memory_callback, 0);
>>> +	for_each_node_state(nid, N_MEMORY)
>>> +		hugetlb_register_node(node_devices[nid]);
>>> +	put_online_mems();
>>> }
>>> #else	/* !CONFIG_NUMA */
>> 
>> Do we really *need* the memory hotplug notifier and the added complexity
>> due for handling memory-less nodes?

Hi David,

After some tries, I think it may not reduce the complexity. node_dev_init()
is called at early stage before hugetlb_register_all_nodes(). So we need to
add a mechanism to detect if the hugetlb subsystem finishes initialization
in node_dev_init() so that it can determine to help hugetlb create /sysfs
files, the mechanism is similar with the changes in drivers/base/node.c of
commit 9a30523066cd ("hugetlb: add per node hstate attributes”). This approach
may add more code than the memory-notify-based approach like this patch
implemented. And it also add the code coupling between node.c and hugetlb.c.
So I tend to use memory hotplug notifier. What’s your opinion?

Thanks,
Muchun

> 
> I have found the commit introduced this mechanism, see commit:
> 
> 	4faf8d950ec4 ("hugetlb: handle memory hot-plug events")
> 
> From the commit message, I think it is a suggestion from David Rientjes.
> I didn’t see any reasons why we need it. So Cc David Rientjes (Maybe
> he knew more context). The committer Lee and the reviewer Andi’s email
> is invalid (don’t Cc them)
> 
>> 
>> Why can't we simply register/unregister sysfs entries in
>> register_node/unregister_node and call it a day?
>> 
> 
> At least, I agree with you. Before I change to this way, let’s wait for
> some potential comments from David Rientjes.
> 
> 
> Thanks.
> 
>> TBH, we should just have sysfs entries for memory-less nodes and not
>> care about such (corner) cases.
>> 
>> 
>> -- 
>> Thanks,
>> 
>> David / dhildenb

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