swapin_readahead: move and rearrange args

swapin_readahead has never sat well in mm/memory.c: move it to mm/swap_state.c
beside its kindred read_swap_cache_async.  Why were its args in a different
order?  rearrange them.  And since it was always followed by a
read_swap_cache_async of the target page, fold that in and return struct
page*.  Then CONFIG_SWAP=n no longer needs valid_swaphandles and
read_swap_cache_async stubs.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/swap_state.c

@@ -10,6 +10,7 @@
 #include <linux/mm.h>
 #include <linux/kernel_stat.h>
 #include <linux/swap.h>
+#include <linux/swapops.h>
 #include <linux/init.h>
 #include <linux/pagemap.h>
 #include <linux/buffer_head.h>
@@ -368,3 +369,49 @@ struct page *read_swap_cache_async(swp_entry_t entry,
 		page_cache_release(new_page);
 	return found_page;
 }
+
+/**
+ * swapin_readahead - swap in pages in hope we need them soon
+ * @entry: swap entry of this memory
+ * @vma: user vma this address belongs to
+ * @addr: target address for mempolicy
+ *
+ * Returns the struct page for entry and addr, after queueing swapin.
+ *
+ * Primitive swap readahead code. We simply read an aligned block of
+ * (1 << page_cluster) entries in the swap area. This method is chosen
+ * because it doesn't cost us any seek time.  We also make sure to queue
+ * the 'original' request together with the readahead ones...
+ *
+ * This has been extended to use the NUMA policies from the mm triggering
+ * the readahead.
+ *
+ * Caller must hold down_read on the vma->vm_mm if vma is not NULL.
+ */
+struct page *swapin_readahead(swp_entry_t entry,
+			struct vm_area_struct *vma, unsigned long addr)
+{
+	int nr_pages;
+	struct page *page;
+	unsigned long offset;
+	unsigned long end_offset;
+
+	/*
+	 * Get starting offset for readaround, and number of pages to read.
+	 * Adjust starting address by readbehind (for NUMA interleave case)?
+	 * No, it's very unlikely that swap layout would follow vma layout,
+	 * more likely that neighbouring swap pages came from the same node:
+	 * so use the same "addr" to choose the same node for each swap read.
+	 */
+	nr_pages = valid_swaphandles(entry, &offset);
+	for (end_offset = offset + nr_pages; offset < end_offset; offset++) {
+		/* Ok, do the async read-ahead now */
+		page = read_swap_cache_async(swp_entry(swp_type(entry), offset),
+						vma, addr);
+		if (!page)
+			break;
+		page_cache_release(page);
+	}
+	lru_add_drain();	/* Push any new pages onto the LRU now */
+	return read_swap_cache_async(entry, vma, addr);
+}