[PATCH 1/3] readahead: return early when readahead is disabled

[Wu Fengguang]

Reduce readahead overheads by returning early in
do_sync_mmap_readahead().

tmpfs has ra_pages=0 and it can page fault really fast
(not constraint by IO if not swapping).

Tested-by: Tim Chen <tim.c.chen@xxxxxxxxx>
Reported-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
 mm/filemap.c |   12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

--- linux-next.orig/mm/filemap.c	2011-04-23 08:56:59.000000000 +0800
+++ linux-next/mm/filemap.c	2011-04-23 09:01:44.000000000 +0800
@@ -1528,6 +1528,8 @@ static void do_sync_mmap_readahead(struc
 	/* If we don't want any read-ahead, don't bother */
 	if (VM_RandomReadHint(vma))
 		return;
+	if (!ra->ra_pages)
+		return;
 
 	if (VM_SequentialReadHint(vma) ||
 			offset - 1 == (ra->prev_pos >> PAGE_CACHE_SHIFT)) {
@@ -1550,12 +1552,10 @@ static void do_sync_mmap_readahead(struc
 	 * mmap read-around
 	 */
 	ra_pages = max_sane_readahead(ra->ra_pages);
-	if (ra_pages) {
-		ra->start = max_t(long, 0, offset - ra_pages/2);
-		ra->size = ra_pages;
-		ra->async_size = 0;
-		ra_submit(ra, mapping, file);
-	}
+	ra->start = max_t(long, 0, offset - ra_pages / 2);
+	ra->size = ra_pages;
+	ra->async_size = 0;
+	ra_submit(ra, mapping, file);
 }
 
 /*


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