bio: add support for inlining a number of bio_vecs inside the bio

When we go and allocate a bio for IO, we actually do two allocations.
One for the bio itself, and one for the bi_io_vec that holds the
actual pages we are interested in.

This feature inlines a definable amount of io vecs inside the bio
itself, so we eliminate the bio_vec array allocation for IO's up
to a certain size. It defaults to 4 vecs, which is typically 16k
of IO.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

diff --git a/fs/bio.c b/fs/bio.c
index 0146f80..75e6be1 100644 (file)
--- a/fs/bio.c
+++ b/fs/bio.c
@@ -31,6 +31,12 @@
 
 DEFINE_TRACE(block_split);
 
+/*
+ * Test patch to inline a certain number of bi_io_vec's inside the bio
+ * itself, to shrink a bio data allocation from two mempool calls to one
+ */
+#define BIO_INLINE_VECS                4
+
 static mempool_t *bio_split_pool __read_mostly;
 
 /*
@@ -241,7 +247,7 @@ void bio_free(struct bio *bio, struct bio_set *bs)
 {
        void *p;
 
-       if (bio->bi_io_vec)
+       if (bio_has_allocated_vec(bio))
                bvec_free_bs(bs, bio->bi_io_vec, BIO_POOL_IDX(bio));
 
        if (bio_integrity(bio))
@@ -267,7 +273,8 @@ static void bio_fs_destructor(struct bio *bio)
 
 static void bio_kmalloc_destructor(struct bio *bio)
 {
-       kfree(bio->bi_io_vec);
+       if (bio_has_allocated_vec(bio))
+               kfree(bio->bi_io_vec);
        kfree(bio);
 }
 
@@ -314,7 +321,16 @@ struct bio *bio_alloc_bioset(gfp_t gfp_mask, int nr_iovecs, struct bio_set *bs)
                if (likely(nr_iovecs)) {
                        unsigned long uninitialized_var(idx);
 
-                       bvl = bvec_alloc_bs(gfp_mask, nr_iovecs, &idx, bs);
+                       if (nr_iovecs <= BIO_INLINE_VECS) {
+                               idx = 0;
+                               bvl = bio->bi_inline_vecs;
+                               nr_iovecs = BIO_INLINE_VECS;
+                               memset(bvl, 0, BIO_INLINE_VECS * sizeof(*bvl));
+                       } else {
+                               bvl = bvec_alloc_bs(gfp_mask, nr_iovecs, &idx,
+                                                       bs);
+                               nr_iovecs = bvec_nr_vecs(idx);
+                       }
                        if (unlikely(!bvl)) {
                                if (bs)
                                        mempool_free(bio, bs->bio_pool);
@@ -324,7 +340,7 @@ struct bio *bio_alloc_bioset(gfp_t gfp_mask, int nr_iovecs, struct bio_set *bs)
                                goto out;
                        }
                        bio->bi_flags |= idx << BIO_POOL_OFFSET;
-                       bio->bi_max_vecs = bvec_nr_vecs(idx);
+                       bio->bi_max_vecs = nr_iovecs;
                }
                bio->bi_io_vec = bvl;
        }
@@ -1525,6 +1541,7 @@ void bioset_free(struct bio_set *bs)
  */
 struct bio_set *bioset_create(unsigned int pool_size, unsigned int front_pad)
 {
+       unsigned int back_pad = BIO_INLINE_VECS * sizeof(struct bio_vec);
        struct bio_set *bs;
 
        bs = kzalloc(sizeof(*bs), GFP_KERNEL);
@@ -1533,7 +1550,7 @@ struct bio_set *bioset_create(unsigned int pool_size, unsigned int front_pad)
 
        bs->front_pad = front_pad;
 
-       bs->bio_slab = bio_find_or_create_slab(front_pad);
+       bs->bio_slab = bio_find_or_create_slab(front_pad + back_pad);
        if (!bs->bio_slab) {
                kfree(bs);
                return NULL;

diff --git a/include/linux/bio.h b/include/linux/bio.h
index 4b80d35..18462c5 100644 (file)
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -102,6 +102,13 @@ struct bio {
 #endif
 
        bio_destructor_t        *bi_destructor; /* destructor */
+
+       /*
+        * We can inline a number of vecs at the end of the bio, to avoid
+        * double allocations for a small number of bio_vecs. This member
+        * MUST obviously be kept at the very end of the bio.
+        */
+       struct bio_vec          bi_inline_vecs[0];
 };
 
 /*
@@ -213,6 +220,11 @@ static inline void *bio_data(struct bio *bio)
        return NULL;
 }
 
+static inline int bio_has_allocated_vec(struct bio *bio)
+{
+       return bio->bi_io_vec && bio->bi_io_vec != bio->bi_inline_vecs;
+}
+
 /*
  * will die
  */