readahead: move synchronous readahead call out of splice loop

Move synchronous page_cache_readahead_ondemand() call out of splice loop.

This avoids one pointless page allocation/insertion in case of non-zero
ra_pages, or many pointless readahead calls in case of zero ra_pages.

Note that if a user sets ra_pages to less than PIPE_BUFFERS=16 pages, he will
not get expected readahead behavior anyway.  The splice code works in batches
of 16 pages, which can be taken as another form of synchronous readahead.

Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
Cc: Jens Axboe <jens.axboe@oracle.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/splice.c

@@ -290,23 +290,24 @@ __generic_file_splice_read(struct file *in, loff_t *ppos,
 	 * Lookup the (hopefully) full range of pages we need.
 	 */
 	spd.nr_pages = find_get_pages_contig(mapping, index, nr_pages, pages);
+	index += spd.nr_pages;
 
 	/*
 	 * If find_get_pages_contig() returned fewer pages than we needed,
-	 * allocate the rest and fill in the holes.
+	 * readahead/allocate the rest and fill in the holes.
 	 */
+	if (spd.nr_pages < nr_pages)
+		page_cache_readahead_ondemand(mapping, &in->f_ra, in,
+				NULL, index, nr_pages - spd.nr_pages);
+
 	error = 0;
-	index += spd.nr_pages;
 	while (spd.nr_pages < nr_pages) {
 		/*
 		 * Page could be there, find_get_pages_contig() breaks on
 		 * the first hole.
 		 */
 		page = find_get_page(mapping, index);
 		if (!page) {
-			page_cache_readahead_ondemand(mapping, &in->f_ra, in,
-					NULL, index, nr_pages - spd.nr_pages);
-
 			/*
 			 * page didn't exist, allocate one.
 			 */