Re: [PATCH v5 1/9] mm/demotion: Add support for explicit memory tiers

[Aneesh Kumar K V]

On 6/8/22 11:40 AM, Ying Huang wrote:
> On Wed, 2022-06-08 at 10:07 +0530, Aneesh Kumar K V wrote:
> > On 6/8/22 12:13 AM, Tim Chen wrote:
> > ...
> >
> > > > +
> > > > +static void memory_tier_device_release(struct device *dev)
> > > > +{
> > > > +	struct memory_tier *tier = to_memory_tier(dev);
> > > > +
> > >
> > > Do we need some ref counts on memory_tier?
> > > If there is another device still using the same memtier,
> > > free below could cause problem.
> > >
> > > > +	kfree(tier);
> > > > +}
> > > > +
> > > ...
> >
> > The lifecycle of the memory_tier struct is tied to the sysfs device life
> > time. ie, memory_tier_device_relese get called only after the last
> > reference on that sysfs dev object is released. Hence we can be sure
> > there is no userspace that is keeping one of the memtier related sysfs
> > file open.
> >
> > W.r.t other memory device sharing the same memtier, we unregister the
> > sysfs device only when the memory tier nodelist is empty. That is no
> > memory device is present in this memory tier.
>
> memory_tier isn't only used by user space.  It is used inside kernel
> too.  If some kernel code get a pointer to struct memory_tier, we need
> to guarantee the pointer will not be freed under us. 

As mentioned above current patchset avoid doing that.

> And as Tim pointed
> out, we need to use it in hot path (for statistics), so some kind of rcu
> lock may be good.

Sure when those statistics code get added, we can add the relevant kref 
and locking details.

-aneesh