[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 38d3e80..dfe242a2 100644
--- a/fs/bio.c
+++ b/fs/bio.c
@@ -325,10 +325,31 @@
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 @@
bio->bi_vcnt++;
bio->bi_phys_segments++;
bio->bi_hw_segments++;
+ done:
bio->bi_size += len;
return len;
}