mm: submit multipage write for SWP_FS_OPS swap-space

swap_writepage() is given one page at a time, but may be called repeatedly
in succession.

For block-device swapspace, the blk_plug functionality allows the multiple
pages to be combined together at lower layers.  That cannot be used for
SWP_FS_OPS as blk_plug may not exist - it is only active when
CONFIG_BLOCK=y.  Consequently all swap reads over NFS are single page
reads.

With this patch we pass a pointer-to-pointer via the wbc.  swap_writepage
can store state between calls - much like the pointer passed explicitly to
swap_readpage.  After calling swap_writepage() some number of times, the
state will be passed to swap_write_unplug() which can submit the combined
request.

Link: https://lkml.kernel.org/r/164859778128.29473.5191868522654408537.stgit@noble.brown
Signed-off-by: NeilBrown <neilb@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: David Howells <dhowells@redhat.com>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Hugh Dickins <hughd@google.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 56 insertions, 22 deletions

diff --git a/mm/page_io.c b/mm/page_io.c
index a63510fd6611..c132511f521c 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -259,8 +259,9 @@ static void sio_write_complete(struct kiocb *iocb, long ret)
 {
 	struct swap_iocb *sio = container_of(iocb, struct swap_iocb, iocb);
 	struct page *page = sio->bvec[0].bv_page;
+	int p;
 
-	if (ret != PAGE_SIZE) {
+	if (ret != PAGE_SIZE * sio->pages) {
 		/*
 		 * In the case of swap-over-nfs, this can be a
 		 * temporary failure if the system has limited
@@ -271,43 +272,63 @@ static void sio_write_complete(struct kiocb *iocb, long ret)
 		 * 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));
+		for (p = 0; p < sio->pages; p++) {
+			page = sio->bvec[p].bv_page;
+			set_page_dirty(page);
+			ClearPageReclaim(page);
+		}
 	} else
-		count_vm_event(PSWPOUT);
-	end_page_writeback(page);
+		count_vm_events(PSWPOUT, sio->pages);
+
+	for (p = 0; p < sio->pages; p++)
+		end_page_writeback(sio->bvec[p].bv_page);
+
 	mempool_free(sio, sio_pool);
 }
 
 static int swap_writepage_fs(struct page *page, struct writeback_control *wbc)
 {
-	struct swap_iocb *sio;
+	struct swap_iocb *sio = NULL;
 	struct swap_info_struct *sis = page_swap_info(page);
 	struct file *swap_file = sis->swap_file;
-	struct address_space *mapping = swap_file->f_mapping;
-	struct iov_iter from;
-	int ret;
+	loff_t pos = page_file_offset(page);
 
 	set_page_writeback(page);
 	unlock_page(page);
-	sio = mempool_alloc(sio_pool, GFP_NOIO);
-	init_sync_kiocb(&sio->iocb, swap_file);
-	sio->iocb.ki_complete = sio_write_complete;
-	sio->iocb.ki_pos = page_file_offset(page);
-	sio->bvec[0].bv_page = page;
-	sio->bvec[0].bv_len = PAGE_SIZE;
-	sio->bvec[0].bv_offset = 0;
-	iov_iter_bvec(&from, WRITE, &sio->bvec[0], 1, PAGE_SIZE);
-	ret = mapping->a_ops->swap_rw(&sio->iocb, &from);
-	if (ret != -EIOCBQUEUED)
-		sio_write_complete(&sio->iocb, ret);
-	return ret;
+	if (wbc->swap_plug)
+		sio = *wbc->swap_plug;
+	if (sio) {
+		if (sio->iocb.ki_filp != swap_file ||
+		    sio->iocb.ki_pos + sio->pages * PAGE_SIZE != pos) {
+			swap_write_unplug(sio);
+			sio = NULL;
+		}
+	}
+	if (!sio) {
+		sio = mempool_alloc(sio_pool, GFP_NOIO);
+		init_sync_kiocb(&sio->iocb, swap_file);
+		sio->iocb.ki_complete = sio_write_complete;
+		sio->iocb.ki_pos = pos;
+		sio->pages = 0;
+	}
+	sio->bvec[sio->pages].bv_page = page;
+	sio->bvec[sio->pages].bv_len = PAGE_SIZE;
+	sio->bvec[sio->pages].bv_offset = 0;
+	sio->pages += 1;
+	if (sio->pages == ARRAY_SIZE(sio->bvec) || !wbc->swap_plug) {
+		swap_write_unplug(sio);
+		sio = NULL;
+	}
+	if (wbc->swap_plug)
+		*wbc->swap_plug = sio;
+
+	return 0;
 }
 
 int __swap_writepage(struct page *page, struct writeback_control *wbc,
-		bio_end_io_t end_write_func)
+		     bio_end_io_t end_write_func)
 {
 	struct bio *bio;
 	int ret;
@@ -344,6 +365,19 @@ int __swap_writepage(struct page *page, struct writeback_control *wbc,
 	return 0;
 }
 
+void swap_write_unplug(struct swap_iocb *sio)
+{
+	struct iov_iter from;
+	struct address_space *mapping = sio->iocb.ki_filp->f_mapping;
+	int ret;
+
+	iov_iter_bvec(&from, WRITE, sio->bvec, sio->pages,
+		      PAGE_SIZE * sio->pages);
+	ret = mapping->a_ops->swap_rw(&sio->iocb, &from);
+	if (ret != -EIOCBQUEUED)
+		sio_write_complete(&sio->iocb, ret);
+}
+
 static void sio_read_complete(struct kiocb *iocb, long ret)
 {
 	struct swap_iocb *sio = container_of(iocb, struct swap_iocb, iocb);