[PATCH] fix bio_add_page for non trivial merge_bvec_fn case

[Dmitri Monakhov <dmonakhov@openvz.org>]

We have to properly decrease all related bio's counters, especially bi_size
in order to merge_bvec_fn return right result. Usually this result in
false merge rejects for two absolutely valid bio_vecs. This may cause 
significant performance penalty for example Itanium: page_size == 16k,
fs_block_size == 1k and block device is raid with small chunk_size.

Signed-off-by: Dmitri Monakhov <dmonakhov@openvz.org>
---
 fs/bio.c |   16 ++++++++++++----
 1 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/fs/bio.c b/fs/bio.c
index 7856257..d713074 100644
--- a/fs/bio.c
+++ b/fs/bio.c
@@ -332,14 +332,21 @@ static int __bio_add_page(struct request_queue *q, struct bio *bio, struct page
 
 		if (page == prev->bv_page &&
 		    offset == prev->bv_offset + prev->bv_len) {
+			/* Temprory detacth last bio_vec. */
+			bio->bi_size -= prev->bv_len;
+			bio->bi_vcnt--;
+			bio->bi_phys_segments--;
+			bio->bi_hw_segments--;
+
 			prev->bv_len += len;
 			if (q->merge_bvec_fn &&
 			    q->merge_bvec_fn(q, bio, prev) < len) {
 				prev->bv_len -= len;
-				return 0;
+				len = 0;
 			}
 
-			goto done;
+			bio->bi_size += prev->bv_len;
+			goto out_add_bvec;
 		}
 	}
 
@@ -394,11 +401,12 @@ static int __bio_add_page(struct request_queue *q, struct bio *bio, struct page
 	    BIOVEC_VIRT_MERGEABLE(bvec-1, bvec)))
 		bio->bi_flags &= ~(1 << BIO_SEG_VALID);
 
+	bio->bi_size += len;
+out_add_bvec:
 	bio->bi_vcnt++;
 	bio->bi_phys_segments++;
 	bio->bi_hw_segments++;
- done:
-	bio->bi_size += len;
 	return len;
 }
 
-- 
1.5.4.rc4