[PATCH v3 6/9] mm: Make __swap_writepage() do async DIO if asked for it

[David Howells]

Make __swap_writepage()'s DIO path do sync DIO if the writeback control's
sync mode is WB_SYNC_ALL and async DIO if not.

Note that this causes hanging processes in sunrpc if the swapfile is on
NFS.  I'm not sure whether it's due to misscheduling or something else.

Suggested-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
cc: Christoph Hellwig <hch@xxxxxx>
cc: Darrick J. Wong <djwong@xxxxxxxxxx>
cc: Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx>
cc: linux-nfs@xxxxxxxxxxxxxxx
cc: linux-block@xxxxxxxxxxxxxxx
cc: linux-xfs@xxxxxxxxxxxxxxx
cc: linux-fsdevel@xxxxxxxxxxxxxxx
cc: linux-mm@xxxxxxxxx
---

 mm/page_io.c |  133 ++++++++++++++++++++++++++++++++++++++++------------------
 1 file changed, 92 insertions(+), 41 deletions(-)

diff --git a/mm/page_io.c b/mm/page_io.c
index 6b1465699c72..8f1199d59162 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -298,6 +298,96 @@ static void bio_associate_blkg_from_page(struct bio *bio, struct page *page)
 #define bio_associate_blkg_from_page(bio, page)		do { } while (0)
 #endif /* CONFIG_MEMCG && CONFIG_BLK_CGROUP */
 
+static void swapfile_write_complete(struct page *page, long ret)
+{
+	if (ret == thp_size(page)) {
+		count_swpout_vm_event(page);
+	} else {
+		/*
+		 * In the case of swap-over-nfs, this can be a
+		 * temporary failure if the system has limited memory
+		 * for allocating transmit buffers.  Mark the page
+		 * dirty and avoid rotate_reclaimable_page but
+		 * rate-limit the messages but do not flag PageError
+		 * like the normal direct-to-bio case as it could be
+		 * temporary.
+		 */
+		set_page_dirty(page);
+		ClearPageReclaim(page);
+		pr_err_ratelimited("Write error (%ld) on dio swapfile (%llu)\n",
+				   ret, page_file_offset(page));
+	}
+	end_page_writeback(page);
+}
+
+static void __swapfile_write_complete(struct kiocb *iocb, long ret, long ret2)
+{
+	struct swapfile_kiocb *ki = container_of(iocb, struct swapfile_kiocb, iocb);
+
+	swapfile_write_complete(iocb->ki_swap_page, ret);
+	swapfile_put_kiocb(ki);
+}
+
+static int swapfile_write_sync(struct swap_info_struct *sis,
+			       struct page *page, struct writeback_control *wbc,
+			       struct iov_iter *from)
+{
+	struct kiocb kiocb;
+	struct file *swap_file = sis->swap_file;
+	int ret;
+
+	init_sync_kiocb(&kiocb, swap_file);
+	kiocb.ki_swap_page	= page;
+	kiocb.ki_pos		= page_file_offset(page);
+	kiocb.ki_flags		= IOCB_DIRECT | IOCB_WRITE | IOCB_SWAP;
+
+	set_page_writeback(page);
+	unlock_page(page);
+
+	ret = swap_file->f_mapping->a_ops->swap_rw(&kiocb, from);
+	swapfile_write_complete(page, ret);
+	return ret == page_size(page) ? 0 : ret >= 0 ? -ENODATA : ret;
+}
+
+static int swapfile_write(struct swap_info_struct *sis,
+			  struct page *page, struct writeback_control *wbc)
+{
+	struct swapfile_kiocb *ki;
+	struct file *swap_file = sis->swap_file;
+	struct bio_vec bv = {
+		.bv_page	= page,
+		.bv_len		= page_size(page),
+		.bv_offset	= 0
+	};
+	struct iov_iter from;
+	int ret;
+
+	iov_iter_bvec(&from, WRITE, &bv, 1, PAGE_SIZE);
+
+	if (wbc->sync_mode == WB_SYNC_ALL)
+		return swapfile_write_sync(sis, page, wbc, &from);
+
+	ki = kzalloc(sizeof(*ki), GFP_KERNEL);
+	if (!ki)
+		return -ENOMEM;
+
+	refcount_set(&ki->ref, 2);
+	init_sync_kiocb(&ki->iocb, swap_file);
+	ki->iocb.ki_swap_page	= page;
+	ki->iocb.ki_pos		= page_file_offset(page);
+	ki->iocb.ki_flags	= IOCB_DIRECT | IOCB_WRITE | IOCB_SWAP;
+	ki->iocb.ki_complete	= __swapfile_write_complete;
+
+	set_page_writeback(page);
+	unlock_page(page);
+	ret = swap_file->f_mapping->a_ops->swap_rw(&ki->iocb, &from);
+
+	if (ret != -EIOCBQUEUED)
+		__swapfile_write_complete(&ki->iocb, ret, 0);
+	swapfile_put_kiocb(ki);
+	return ret == page_size(page) ? 0 : ret >= 0 ? -ENODATA : ret;
+}
+
 int __swap_writepage(struct page *page, struct writeback_control *wbc)
 {
 	struct bio *bio;
@@ -305,47 +395,8 @@ int __swap_writepage(struct page *page, struct writeback_control *wbc)
 	struct swap_info_struct *sis = page_swap_info(page);
 
 	VM_BUG_ON_PAGE(!PageSwapCache(page), page);
-	if (data_race(sis->flags & SWP_FS_OPS)) {
-		struct kiocb kiocb;
-		struct file *swap_file = sis->swap_file;
-		struct address_space *mapping = swap_file->f_mapping;
-		struct bio_vec bv = {
-			.bv_page = page,
-			.bv_len  = PAGE_SIZE,
-			.bv_offset = 0
-		};
-		struct iov_iter from;
-
-		iov_iter_bvec(&from, WRITE, &bv, 1, PAGE_SIZE);
-		init_sync_kiocb(&kiocb, swap_file);
-		kiocb.ki_pos	= page_file_offset(page);
-		kiocb.ki_flags	= IOCB_DIRECT | IOCB_WRITE | IOCB_SWAP;
-
-		set_page_writeback(page);
-		unlock_page(page);
-		ret = mapping->a_ops->swap_rw(&kiocb, &from);
-		if (ret == PAGE_SIZE) {
-			count_vm_event(PSWPOUT);
-			ret = 0;
-		} else {
-			/*
-			 * In the case of swap-over-nfs, this can be a
-			 * temporary failure if the system has limited
-			 * memory for allocating transmit buffers.
-			 * Mark the page dirty and avoid
-			 * rotate_reclaimable_page but rate-limit the
-			 * messages but do not flag PageError like
-			 * the normal direct-to-bio case as it could
-			 * be temporary.
-			 */
-			set_page_dirty(page);
-			ClearPageReclaim(page);
-			pr_err_ratelimited("Write error (%d) on dio swapfile (%llu)\n",
-					   ret, page_file_offset(page));
-		}
-		end_page_writeback(page);
-		return ret;
-	}
+	if (data_race(sis->flags & SWP_FS_OPS))
+		return swapfile_write(sis, page, wbc);
 
 	ret = bdev_write_page(sis->bdev, swap_page_sector(page), page, wbc);
 	if (!ret) {