[PATCH v3 hmm 04/12] mm/hmm: Simplify hmm_get_or_create and make it reliable

[Christoph Hellwig]

> +	spin_lock(&mm->page_table_lock);
> +	if (mm->hmm) {
> +		if (kref_get_unless_zero(&mm->hmm->kref)) {
> +			spin_unlock(&mm->page_table_lock);
> +			return mm->hmm;
> +		}
> +	}
> +	spin_unlock(&mm->page_table_lock);

This could become:

	spin_lock(&mm->page_table_lock);
	hmm = mm->hmm
	if (hmm && kref_get_unless_zero(&hmm->kref))
		goto out_unlock;
	spin_unlock(&mm->page_table_lock);

as the last two lines of the function already drop the page_table_lock
and then return hmm.  Or drop the "hmm = mm->hmm" asignment above and
return mm->hmm as that should be always identical to hmm at the end
to save another line.

> +	/*
> +	 * The mm->hmm pointer is kept valid while notifier ops can be running
> +	 * so they don't have to deal with a NULL mm->hmm value
> +	 */

The comment confuses me.  How does the page_table_lock relate to
possibly running notifiers, as I can't find that we take
page_table_lock?  Or is it just about the fact that we only clear
mm->hmm in the free callback, and not in hmm_free?