[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>

diff --git a/fs/bio.c b/fs/bio.c
index 38d3e8023a0795273595b46ef8647ebaee9952e6..dfe242a21eb4cb49d00d4ee54def5c024be1ea0b 100644 (file)
--- 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;
 }