page cache: Convert page deletion to XArray

 *   ->tasklist_lock            (memory_failure, collect_procs_ao)

 */

static void page_cache_tree_delete(struct address_space *mapping,

static void page_cache_delete(struct address_space *mapping,

				   struct page *page, void *shadow)

{

	int i, nr;

	XA_STATE(xas, &mapping->i_pages, page->index);

	unsigned int nr = 1;

	/* hugetlb pages are represented by one entry in the radix tree */

	nr = PageHuge(page) ? 1 : hpage_nr_pages(page);

	mapping_set_update(&xas, mapping);

	/* hugetlb pages are represented by a single entry in the xarray */

	if (!PageHuge(page)) {

		xas_set_order(&xas, page->index, compound_order(page));

		nr = 1U << compound_order(page);

	}

	VM_BUG_ON_PAGE(!PageLocked(page), page);

	VM_BUG_ON_PAGE(PageTail(page), page);

	VM_BUG_ON_PAGE(nr != 1 && shadow, page);

	for (i = 0; i < nr; i++) {

		struct radix_tree_node *node;

		void **slot;

		__radix_tree_lookup(&mapping->i_pages, page->index + i,

				    &node, &slot);

		VM_BUG_ON_PAGE(!node && nr != 1, page);

		radix_tree_clear_tags(&mapping->i_pages, node, slot);

		__radix_tree_replace(&mapping->i_pages, node, slot, shadow,

				workingset_lookup_update(mapping));

	}

	xas_store(&xas, shadow);

	xas_init_marks(&xas);

	page->mapping = NULL;

	/* Leave page->index set: truncation lookup relies upon it */

	trace_mm_filemap_delete_from_page_cache(page);

	unaccount_page_cache_page(mapping, page);

	page_cache_tree_delete(mapping, page, shadow);

	page_cache_delete(mapping, page, shadow);

}

static void page_cache_free_page(struct address_space *mapping,