mm: move readahead nr_pages check into read_pages

Simplify the callers by moving the check for nr_pages and the BUG_ON
into read_pages().

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Cc: Chao Yu <yuchao0@huawei.com>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Darrick J. Wong <darrick.wong@oracle.com>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: Eric Biggers <ebiggers@google.com>
Cc: Gao Xiang <gaoxiang25@huawei.com>
Cc: Jaegeuk Kim <jaegeuk@kernel.org>
Cc: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Miklos Szeredi <mszeredi@redhat.com>
Link: http://lkml.kernel.org/r/20200414150233.24495-5-willy@infradead.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/readahead.c

@@ -119,6 +119,9 @@ static void read_pages(struct address_space *mapping, struct file *filp,
 	struct blk_plug plug;
 	unsigned page_idx;
 
+	if (!nr_pages)
+		return;
+
 	blk_start_plug(&plug);
 
 	if (mapping->a_ops->readpages) {
@@ -138,6 +141,8 @@ static void read_pages(struct address_space *mapping, struct file *filp,
 
 out:
 	blk_finish_plug(&plug);
+
+	BUG_ON(!list_empty(pages));
 }
 
 /*
@@ -180,8 +185,7 @@ void __do_page_cache_readahead(struct address_space *mapping,
 			 * contiguous pages before continuing with the next
 			 * batch.
 			 */
-			if (nr_pages)
-				read_pages(mapping, filp, &page_pool, nr_pages,
+			read_pages(mapping, filp, &page_pool, nr_pages,
 						gfp_mask);
 			nr_pages = 0;
 			continue;
@@ -202,9 +206,7 @@ void __do_page_cache_readahead(struct address_space *mapping,
 	 * uptodate then the caller will launch readpage again, and
 	 * will then handle the error.
 	 */
-	if (nr_pages)
-		read_pages(mapping, filp, &page_pool, nr_pages, gfp_mask);
-	BUG_ON(!list_empty(&page_pool));
+	read_pages(mapping, filp, &page_pool, nr_pages, gfp_mask);
 }
 
 /*