Btrfs: finish read pages in the order they are submitted

The endio is done at reverse order of bio vectors.

That means for a sequential read, the page first submitted will finish
last in a bio. Considering we will do checksum (making cache hot) for
every page, this does introduce delay (and chance to squeeze cache used
soon) for pages submitted at the begining.

I don't observe obvious performance difference with below patch at my
simple test, but seems more natural to finish read in the order they are
submitted.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 4 insertions, 3 deletions

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 7073cbb1b2d4..355a973719a0 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -1750,7 +1750,8 @@ static void end_bio_extent_writepage(struct bio *bio, int err)
 static void end_bio_extent_readpage(struct bio *bio, int err)
 {
         int uptodate = test_bit(BIO_UPTODATE, &bio->bi_flags);
-        struct bio_vec *bvec = bio->bi_io_vec + bio->bi_vcnt - 1;
+        struct bio_vec *bvec_end = bio->bi_io_vec + bio->bi_vcnt - 1;
+        struct bio_vec *bvec = bio->bi_io_vec;
         struct extent_io_tree *tree;
         u64 start;
         u64 end;
@@ -1773,7 +1774,7 @@ static void end_bio_extent_readpage(struct bio *bio, int err)
                 else
                         whole_page = 0;
 
-                if (--bvec >= bio->bi_io_vec)
+                if (++bvec <= bvec_end)
                         prefetchw(&bvec->bv_page->flags);
 
                 if (uptodate && tree->ops && tree->ops->readpage_end_io_hook) {
@@ -1818,7 +1819,7 @@ static void end_bio_extent_readpage(struct bio *bio, int err)
                         }
                         check_page_locked(tree, page);
                 }
-        } while (bvec >= bio->bi_io_vec);
+        } while (bvec <= bvec_end);
 
         bio_put(bio);
 }