Re: [PATCH 01/11] mm,migration: Take a reference to the anon_vma before migrating

[KOSAKI Motohiro]

>  rcu_unlock:
> +
> +	/* Drop an anon_vma reference if we took one */
> +	if (anon_vma && atomic_dec_and_lock(&anon_vma->migrate_refcount, &anon_vma->lock)) {
> +		int empty = list_empty(&anon_vma->head);
> +		spin_unlock(&anon_vma->lock);
> +		if (empty)
> +			anon_vma_free(anon_vma);
> +	}
> +

Why don't we check ksm_refcount here? Also, why drop_anon_vma() doesn't
need check migrate_refcount?

plus, if we add this logic, we can remove SLAB_DESTROY_BY_RCU from 
anon_vma_cachep and rcu_read_lock() from unmap_and_move(), I think.
It is for preventing anon_vma recycle logic. but no free directly mean
no memory recycle.



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