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

[Dan Williams <dan.j.williams@intel.com>]

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@sk.com>
> > Signed-off-by: Honggyu Kim <honggyu.kim@sk.com>
> > Signed-off-by: Yunjeong Mun <yunjeong.mun@sk.com>
> > Reviewed-by: Gregory Price <gourry@gourry.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.