Re: [PATCH 29/29] mm: shrinker: move shrinker-related code into a separate file

[Qi Zheng]

Hi Sergey,

On 2023/6/23 13:25, Sergey Senozhatsky wrote:
> On (23/06/22 16:53), Qi Zheng wrote:
> > +/*
> > + * Remove one
> > + */
> > +void unregister_shrinker(struct shrinker *shrinker)
> > +{
> > +	struct dentry *debugfs_entry;
> > +	int debugfs_id;
> > +
> > +	if (!(shrinker->flags & SHRINKER_REGISTERED))
> > +		return;
> > +
> > +	shrinker_put(shrinker);
> > +	wait_for_completion(&shrinker->completion_wait);
> > +
> > +	mutex_lock(&shrinker_mutex);
> > +	list_del_rcu(&shrinker->list);
>
> Should this function wait for RCU grace period(s) before it goes
> touching shrinker fields?

Why? We will free this shrinker instance by rcu after executing
unregister_shrinker(). So it is safe to touch shrinker fields here.

> > +	shrinker->flags &= ~SHRINKER_REGISTERED;
> > +	if (shrinker->flags & SHRINKER_MEMCG_AWARE)
> > +		unregister_memcg_shrinker(shrinker);
> > +	debugfs_entry = shrinker_debugfs_detach(shrinker, &debugfs_id);
> > +	mutex_unlock(&shrinker_mutex);
> > +
> > +	shrinker_debugfs_remove(debugfs_entry, debugfs_id);
> > +
> > +	kfree(shrinker->nr_deferred);
> > +	shrinker->nr_deferred = NULL;
> > +}
> > +EXPORT_SYMBOL(unregister_shrinker);
>
> [..]
>
> > +void shrinker_free(struct shrinker *shrinker)
> > +{
> > +	kfree(shrinker);
> > +}
> > +EXPORT_SYMBOL(shrinker_free);
> > +
> > +void unregister_and_free_shrinker(struct shrinker *shrinker)
> > +{
> > +	unregister_shrinker(shrinker);
> > +	kfree_rcu(shrinker, rcu);
> > +}
>
> Seems like this
>
> 	unregister_shrinker();
> 	shrinker_free();
>
> is not exact equivalent of this
>
> 	unregister_and_free_shrinker();

Yes, my original intention is that shrinker_free() is only used to
handle the case where register_shrinker() returns failure.

I will implement the method suggested by Dave in 02/29. Those APIs are
more concise and will bring more benefits. :)

Thanks,
Qi