MM: handle THP in swap_*page_fs() - count_vm_events()

We need to use count_swpout_vm_event() for sio_write_complete() to get
correct counting.

Note that THP swap in (if it ever happens) is current accounted 1 for each
page, whether HUGE or normal.  This is different from swap-out accounting.

This patch should be squashed into
    MM: handle THP in swap_*page_fs()

Link: https://lkml.kernel.org/r/165146948934.24404.5909750610552745025@noble.neil.brown.nameSigned-off-by: NeilBrown <neilb@suse.de>
Reported-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Huang Ying <ying.huang@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/page_io.c

@@ -280,8 +280,10 @@ static void sio_write_complete(struct kiocb *iocb, long ret)
 			set_page_dirty(page);
 			ClearPageReclaim(page);
 		}
-	} else
-		count_vm_events(PSWPOUT, sio->pages);
+	} else {
+		for (p = 0; p < sio->pages; p++)
+			count_swpout_vm_event(sio->bvec[p].bv_page);
+	}
 
 	for (p = 0; p < sio->pages; p++)
 		end_page_writeback(sio->bvec[p].bv_page);