readahead: move the random read case to bottom

Split all readahead cases, and move the random one to bottom.

No behavior changes.

This is to prepare for the introduction of context readahead, and make it
easy for inserting accounting/tracing points for each case.
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Cc: Vladislav Bolkhovitin <vst@vlnb.net>
Cc: Jens Axboe <jens.axboe@oracle.com>
Cc: Jeff Moyer <jmoyer@redhat.com>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Ying Han <yinghan@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/readahead.c

@@ -339,14 +339,18 @@ ondemand_readahead(struct address_space *mapping,
 		   unsigned long req_size)
 {
 	unsigned long max = max_sane_readahead(ra->ra_pages);
-	pgoff_t prev_offset;
-	int	sequential;
+
+	/*
+	 * start of file
+	 */
+	if (!offset)
+		goto initial_readahead;
 
 	/*
 	 * It's the expected callback offset, assume sequential access.
 	 * Ramp up sizes, and push forward the readahead window.
 	 */
-	if (offset && (offset == (ra->start + ra->size - ra->async_size) ||
+	if ((offset == (ra->start + ra->size - ra->async_size) ||
 	     offset == (ra->start + ra->size))) {
 		ra->start += ra->size;
 		ra->size = get_next_ra_size(ra, max);
@@ -354,18 +358,6 @@ ondemand_readahead(struct address_space *mapping,
 		goto readit;
 	}
 
-	prev_offset = ra->prev_pos >> PAGE_CACHE_SHIFT;
-	sequential = offset - prev_offset <= 1UL || req_size > max;
-
-	/*
-	 * Standalone, small read.
-	 * Read as is, and do not pollute the readahead state.
-	 */
-	if (!hit_readahead_marker && !sequential) {
-		return __do_page_cache_readahead(mapping, filp,
-						offset, req_size, 0);
-	}
-
 	/*
 	 * Hit a marked page without valid readahead state.
 	 * E.g. interleaved reads.
@@ -391,12 +383,24 @@ ondemand_readahead(struct address_space *mapping,
 	}
 
 	/*
-	 * It may be one of
-	 * 	- first read on start of file
-	 * 	- sequential cache miss
-	 * 	- oversize random read
-	 * Start readahead for it.
+	 * oversize read
 	 */
+	if (req_size > max)
+		goto initial_readahead;
+
+	/*
+	 * sequential cache miss
+	 */
+	if (offset - (ra->prev_pos >> PAGE_CACHE_SHIFT) <= 1UL)
+		goto initial_readahead;
+
+	/*
+	 * standalone, small random read
+	 * Read as is, and do not pollute the readahead state.
+	 */
+	return __do_page_cache_readahead(mapping, filp, offset, req_size, 0);
+
+initial_readahead:
 	ra->start = offset;
 	ra->size = get_init_ra_size(req_size, max);
 	ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;