fix-prefetchw-of-NULL-page

   1 ext4: fix prefetchw of NULL page
   2
   3 From: Liu Xiang <liu.xiang6@zte.com.cn>
   4
   5 In ext4_mpage_readpages(), if the parameter pages is not NULL, another
   6 parameter page is NULL. At the first time prefetchw(&page->flags)
   7 works on NULL. From second time, prefetchw(&page->flags) always works on
   8 the last consumed page. This might do little improvment for handling
   9 current page. So prefetchw() should be called while the page pointer
  10 has just been updated.
  11
  12 Signed-off-by: Liu Xiang <liu.xiang6@zte.com.cn>
  13 Signed-off-by: Theodore Ts'o <tytso@mit.edu>
  14 ---
  15
  16 Changes in v2:
  17  remove prefetchw() when pages is NULL and change comment
  18
  19  fs/ext4/readpage.c | 3 ++-
  20  1 file changed, 2 insertions(+), 1 deletion(-)
  21
  22 diff --git a/fs/ext4/readpage.c b/fs/ext4/readpage.c
  23 index 6aa282e..f138c47 100644
  24 --- a/fs/ext4/readpage.c
  25 +++ b/fs/ext4/readpage.c
  26 @@ -126,9 +126,10 @@ int ext4_mpage_readpages(struct address_space *mapping,
  27                 int fully_mapped = 1;
  28                 unsigned first_hole = blocks_per_page;
  29
  30 -               prefetchw(&page->flags);
  31                 if (pages) {
  32                         page = lru_to_page(pages);
  33 +
  34 +                       prefetchw(&page->flags);
  35                         list_del(&page->lru);
  36                         if (add_to_page_cache_lru(page, mapping, page->index,
  37                                   readahead_gfp_mask(mapping)))
  38 --
  39 1.9.1
  40
  41