readahead: return early when readahead is disabled

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).

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Tested-by: Tim Chen <tim.c.chen@intel.com>
Reported-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 6 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index 88354ae0b1fd..c974a2863897 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1556,6 +1556,8 @@ static void do_sync_mmap_readahead(struct vm_area_struct *vma,
         /* 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)) {
@@ -1578,12 +1580,10 @@ static void do_sync_mmap_readahead(struct vm_area_struct *vma,
          * 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);
 }
 
 /*