linux-kernel - Re: [PATCH v7 2/3] mm/mempolicy: Prepare weighted interleave sysfs for memory hotplug

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Message-ID: <20250409055618.512-1-rakie.kim@sk.com>
Date: Wed,  9 Apr 2025 14:56:11 +0900
From: Rakie Kim <rakie.kim@...com>
To: Dan Williams <dan.j.williams@...el.com>
Cc: gourry@...rry.net,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	linux-cxl@...r.kernel.org,
	joshua.hahnjy@...il.com,
	ying.huang@...ux.alibaba.com,
	david@...hat.com,
	Jonathan.Cameron@...wei.com,
	osalvador@...e.de,
	kernel_team@...ynix.com,
	honggyu.kim@...com,
	yunjeong.mun@...com,
	rakie.kim@...com,
	akpm@...ux-foundation.org
Subject: Re: [PATCH v7 2/3] mm/mempolicy: Prepare weighted interleave sysfs for memory hotplug

On Tue, 8 Apr 2025 20:54:48 -0700 Dan Williams <dan.j.williams@...el.com> wrote:
> Dan Williams wrote:
> > Rakie Kim wrote:
> > > Previously, the weighted interleave sysfs structure was statically
> > > managed during initialization. This prevented new nodes from being
> > > recognized when memory hotplug events occurred, limiting the ability
> > > to update or extend sysfs entries dynamically at runtime.
> > > 
> > > To address this, this patch refactors the sysfs infrastructure and
> > > encapsulates it within a new structure, `sysfs_wi_group`, which holds
> > > both the kobject and an array of node attribute pointers.
> > > 
> > > By allocating this group structure globally, the per-node sysfs
> > > attributes can be managed beyond initialization time, enabling
> > > external modules to insert or remove node entries in response to
> > > events such as memory hotplug or node online/offline transitions.
> > > 
> > > Instead of allocating all per-node sysfs attributes at once, the
> > > initialization path now uses the existing sysfs_wi_node_add() and
> > > sysfs_wi_node_delete() helpers. This refactoring makes it possible
> > > to modularly manage per-node sysfs entries and ensures the
> > > infrastructure is ready for runtime extension.
> > > 
> > > Signed-off-by: Rakie Kim <rakie.kim@...com>
> > > Signed-off-by: Honggyu Kim <honggyu.kim@...com>
> > > Signed-off-by: Yunjeong Mun <yunjeong.mun@...com>
> > > Reviewed-by: Gregory Price <gourry@...rry.net>
> > > ---
> > >  mm/mempolicy.c | 61 ++++++++++++++++++++++++--------------------------
> > >  1 file changed, 29 insertions(+), 32 deletions(-)
> > > 
> > > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> > > index 0da102aa1cfc..988575f29c53 100644
> > > --- a/mm/mempolicy.c
> > > +++ b/mm/mempolicy.c
> > > @@ -3419,6 +3419,13 @@ struct iw_node_attr {
> > >  	int nid;
> > >  };
> > >  
> > > +struct sysfs_wi_group {
> > > +	struct kobject wi_kobj;
> > > +	struct iw_node_attr *nattrs[];
> > > +};
> > > +
> > > +static struct sysfs_wi_group *wi_group;
> > > +
> > >  static ssize_t node_show(struct kobject *kobj, struct kobj_attribute *attr,
> > >  			 char *buf)
> > >  {
> > > @@ -3461,27 +3468,24 @@ static ssize_t node_store(struct kobject *kobj, struct kobj_attribute *attr,
> > >  	return count;
> > >  }
> > >  
> > > -static struct iw_node_attr **node_attrs;
> > > -
> > > -static void sysfs_wi_node_release(struct iw_node_attr *node_attr,
> > > -				  struct kobject *parent)
> > > +static void sysfs_wi_node_delete(int nid)
> > >  {
> > > -	if (!node_attr)
> > > +	if (!wi_group->nattrs[nid])
> > >  		return;
> > > -	sysfs_remove_file(parent, &node_attr->kobj_attr.attr);
> > > -	kfree(node_attr->kobj_attr.attr.name);
> > > -	kfree(node_attr);
> > > +
> > > +	sysfs_remove_file(&wi_group->wi_kobj,
> > > +			  &wi_group->nattrs[nid]->kobj_attr.attr);
> > 
> > This still looks broken to me, but I think this is more a problem that
> > was present in the original code.
> > 
> > At this point @wi_group's reference count is zero because
> > sysfs_wi_release() has been called. However, it can only be zero if it has
> > properly transitioned through kobject_del() and final kobject_put(). It
> > follows that kobject_del() arranges for kobj->sd to be NULL. That means
> > that this *should* be hitting the WARN() in kernfs_remove_by_name_ns()
> > for the !parent case.
> > 
> > So, either you are not triggering that path, or testing that path, but
> > sys_remove_file() of the child attributes should be happening *before*
> > sysfs_wi_release().
> > 
> > Did I miss something?
> 
> I think the missing change is that sysfs_wi_node_add() failures need to
> be done with a sysfs_wi_node_delete() of the added attrs *before* the
> kobject_del() of @wi_group.

Hi Dan Williams

Thank you very much for identifying this potential issue in the code.

As you pointed out, this seems to be a problem that was already present in
the original implementation, and I agree that it needs to be addressed.

However, since this issue existed prior to the changes in this patch
series, I believe it would be more appropriate to fix it in a separate
follow-up patch rather than include it here.

I will start preparing a new patch to address this problem, and I would
greatly appreciate it if you could review it once it's ready.

Rakie