ext4: readpages() should submit IO as read-ahead

a_ops->readpages() is only ever used for read-ahead.  Ensure that we
pass this information down to the block layer.

Link: http://lkml.kernel.org/r/20180621010725.17813-5-axboe@kernel.dk
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Chris Mason <clm@fb.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/ext4/ext4.h

@@ -3062,7 +3062,7 @@ static inline void ext4_set_de_type(struct super_block *sb,
 /* readpages.c */
 extern int ext4_mpage_readpages(struct address_space *mapping,
 				struct list_head *pages, struct page *page,
-				unsigned nr_pages);
+				unsigned nr_pages, bool is_readahead);
 
 /* symlink.c */
 extern const struct inode_operations ext4_encrypted_symlink_inode_operations;

fs/ext4/inode.c

@@ -3325,7 +3325,8 @@ static int ext4_readpage(struct file *file, struct page *page)
 		ret = ext4_readpage_inline(inode, page);
 
 	if (ret == -EAGAIN)
-		return ext4_mpage_readpages(page->mapping, NULL, page, 1);
+		return ext4_mpage_readpages(page->mapping, NULL, page, 1,
+						false);
 
 	return ret;
 }
@@ -3340,7 +3341,7 @@ ext4_readpages(struct file *file, struct address_space *mapping,
 	if (ext4_has_inline_data(inode))
 		return 0;
 
-	return ext4_mpage_readpages(mapping, pages, NULL, nr_pages);
+	return ext4_mpage_readpages(mapping, pages, NULL, nr_pages, true);
 }
 
 static void ext4_invalidatepage(struct page *page, unsigned int offset,

fs/ext4/readpage.c

@@ -98,7 +98,7 @@ static void mpage_end_io(struct bio *bio)
 
 int ext4_mpage_readpages(struct address_space *mapping,
 			 struct list_head *pages, struct page *page,
-			 unsigned nr_pages)
+			 unsigned nr_pages, bool is_readahead)
 {
 	struct bio *bio = NULL;
 	sector_t last_block_in_bio = 0;
@@ -259,7 +259,8 @@ int ext4_mpage_readpages(struct address_space *mapping,
 			bio->bi_iter.bi_sector = blocks[0] << (blkbits - 9);
 			bio->bi_end_io = mpage_end_io;
 			bio->bi_private = ctx;
-			bio_set_op_attrs(bio, REQ_OP_READ, 0);
+			bio_set_op_attrs(bio, REQ_OP_READ,
+						is_readahead ? REQ_RAHEAD : 0);
 		}
 
 		length = first_hole << blkbits;