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Date:   Wed, 3 Apr 2019 10:23:26 -0700
From:   Matthew Wilcox <willy@infradead.org>
To:     "Tobin C. Harding" <tobin@kernel.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Roman Gushchin <guro@fb.com>,
        Alexander Viro <viro@ftp.linux.org.uk>,
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Subject: Re: [RFC PATCH v2 09/14] xarray: Implement migration function for
 objects
Message-ID: <20190403172326.GJ22763@bombadil.infradead.org>
References: <20190403042127.18755-1-tobin@kernel.org>
 <20190403042127.18755-10-tobin@kernel.org>
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On Wed, Apr 03, 2019 at 03:21:22PM +1100, Tobin C. Harding wrote:
> +void xa_object_migrate(struct xa_node *node, int numa_node)
> +{
> +	struct xarray *xa = READ_ONCE(node->array);
> +	void __rcu **slot;
> +	struct xa_node *new_node;
> +	int i;
> +
> +	/* Freed or not yet in tree then skip */
> +	if (!xa || xa == XA_RCU_FREE)
> +		return;
> +
> +	new_node = kmem_cache_alloc_node(radix_tree_node_cachep,
> +					 GFP_KERNEL, numa_node);
> +	if (!new_node)
> +		return;
> +
> +	xa_lock_irq(xa);
> +
> +	/* Check again..... */
> +	if (xa != node->array || !list_empty(&node->private_list)) {
> +		node = new_node;
> +		goto unlock;
> +	}
> +
> +	memcpy(new_node, node, sizeof(struct xa_node));
> +
> +	/* Move pointers to new node */
> +	INIT_LIST_HEAD(&new_node->private_list);

Surely we can do something more clever, like ...

	if (xa != node->array) {
...
	if (list_empty(&node->private_list))
		INIT_LIST_HEAD(&new_node->private_list);
	else
		list_replace(&node->private_list, &new_node->private_list);


BTW, the raidx tree nodes / xa_nodes share the same slab cache; we need
to finish converting all radix tree & IDR users to the XArray before
this series can go in.