swap: use bdev_read_page() / bdev_write_page()

By calling the device driver to write the page directly, we avoid
allocating a BIO, which allows us to free memory without allocating
memory.

[akpm@linux-foundation.org: fix used-uninitialized bug]
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Dheeraj Reddy <dheeraj.reddy@intel.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_io.c

@@ -248,11 +248,16 @@ out:
 	return ret;
 }
 
+static sector_t swap_page_sector(struct page *page)
+{
+	return (sector_t)__page_file_index(page) << (PAGE_CACHE_SHIFT - 9);
+}
+
 int __swap_writepage(struct page *page, struct writeback_control *wbc,
 	void (*end_write_func)(struct bio *, int))
 {
 	struct bio *bio;
-	int ret = 0, rw = WRITE;
+	int ret, rw = WRITE;
 	struct swap_info_struct *sis = page_swap_info(page);
 
 	if (sis->flags & SWP_FILE) {
@@ -297,6 +302,13 @@ int __swap_writepage(struct page *page, struct writeback_control *wbc,
 		return ret;
 	}
 
+	ret = bdev_write_page(sis->bdev, swap_page_sector(page), page, wbc);
+	if (!ret) {
+		count_vm_event(PSWPOUT);
+		return 0;
+	}
+
+	ret = 0;
 	bio = get_swap_bio(GFP_NOIO, page, end_write_func);
 	if (bio == NULL) {
 		set_page_dirty(page);
@@ -338,6 +350,13 @@ int swap_readpage(struct page *page)
 		return ret;
 	}
 
+	ret = bdev_read_page(sis->bdev, swap_page_sector(page), page);
+	if (!ret) {
+		count_vm_event(PSWPIN);
+		return 0;
+	}
+
+	ret = 0;
 	bio = get_swap_bio(GFP_KERNEL, page, end_swap_bio_read);
 	if (bio == NULL) {
 		unlock_page(page);