[BLOCK] bio: check for same page merge possibilities in __bio_add_page()

For filesystems with a blocksize < page size, we can merge same page
calls into the bio_vec at the end of the bio. This saves segments
on systems with a page size > the "normal" 4kb fs block size.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@suse.de>

1 files changed, 24 insertions, 2 deletions

diff --git a/fs/bio.c b/fs/bio.c
index 38d3e8023a07..dfe242a21eb4 100644
--- a/fs/bio.c
+++ b/fs/bio.c
@@ -325,10 +325,31 @@ static int __bio_add_page(request_queue_t *q, struct bio *bio, struct page
         if (unlikely(bio_flagged(bio, BIO_CLONED)))
                 return 0;
 
-        if (bio->bi_vcnt >= bio->bi_max_vecs)
+        if (((bio->bi_size + len) >> 9) > max_sectors)
                 return 0;
 
-        if (((bio->bi_size + len) >> 9) > max_sectors)
+        /*
+         * For filesystems with a blocksize smaller than the pagesize
+         * we will often be called with the same page as last time and
+         * a consecutive offset.  Optimize this special case.
+         */
+        if (bio->bi_vcnt > 0) {
+                struct bio_vec *prev = &bio->bi_io_vec[bio->bi_vcnt - 1];
+
+                if (page == prev->bv_page &&
+                    offset == prev->bv_offset + prev->bv_len) {
+                        prev->bv_len += len;
+                        if (q->merge_bvec_fn &&
+                            q->merge_bvec_fn(q, bio, prev) < len) {
+                                prev->bv_len -= len;
+                                return 0;
+                        }
+
+                        goto done;
+                }
+        }
+
+        if (bio->bi_vcnt >= bio->bi_max_vecs)
                 return 0;
 
         /*
@@ -382,6 +403,7 @@ static int __bio_add_page(request_queue_t *q, struct bio *bio, struct page
         bio->bi_vcnt++;
         bio->bi_phys_segments++;
         bio->bi_hw_segments++;
+ done:
         bio->bi_size += len;
         return len;
 }