block: Add bio_reset()

Reusing bios is something that's been highly frowned upon in the past,
but driver code keeps doing it anyways. If it's going to happen anyways,
we should provide a generic method.

This'll help with getting rid of bi_destructor - drivers/block/pktcdvd.c
was open coding it, by doing a bio_init() and resetting bi_destructor.

This required reordering struct bio, but the block layer is not yet
nearly fast enough for any cacheline effects to matter here.

v5: Add a define BIO_RESET_BITS, to be very explicit about what parts of
bio->bi_flags are saved.
v6: Further commenting verbosity, per Tejun
v9: Add a function comment

Signed-off-by: Kent Overstreet <koverstreet@google.com>
CC: Jens Axboe <axboe@kernel.dk>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

3 files changed, 44 insertions, 6 deletions

diff --git a/fs/bio.c b/fs/bio.c
index b14f71adff4a..919ee9aa5c57 100644
--- a/fs/bio.c
+++ b/fs/bio.c
@@ -263,6 +263,30 @@ void bio_init(struct bio *bio)
 EXPORT_SYMBOL(bio_init);
 
 /**
+ * bio_reset - reinitialize a bio
+ * @bio:        bio to reset
+ *
+ * Description:
+ *   After calling bio_reset(), @bio will be in the same state as a freshly
+ *   allocated bio returned bio bio_alloc_bioset() - the only fields that are
+ *   preserved are the ones that are initialized by bio_alloc_bioset(). See
+ *   comment in struct bio.
+ */
+void bio_reset(struct bio *bio)
+{
+        unsigned long flags = bio->bi_flags & (~0UL << BIO_RESET_BITS);
+
+        if (bio_integrity(bio))
+                bio_integrity_free(bio);
+
+        bio_disassociate_task(bio);
+
+        memset(bio, 0, BIO_RESET_BYTES);
+        bio->bi_flags = flags|(1 << BIO_UPTODATE);
+}
+EXPORT_SYMBOL(bio_reset);
+
+/**
  * bio_alloc_bioset - allocate a bio for I/O
  * @gfp_mask:   the GFP_ mask given to the slab allocator
  * @nr_iovecs:  number of iovecs to pre-allocate
diff --git a/include/linux/bio.h b/include/linux/bio.h
index a11f74bc82d2..76f6c252baff 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -226,6 +226,7 @@ extern void __bio_clone(struct bio *, struct bio *);
 extern struct bio *bio_clone(struct bio *, gfp_t);
 
 extern void bio_init(struct bio *);
+extern void bio_reset(struct bio *);
 
 extern int bio_add_page(struct bio *, struct page *, unsigned int,unsigned int);
 extern int bio_add_pc_page(struct request_queue *, struct bio *, struct page *,
diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
index af9dd9d2efc4..1b607c247d72 100644
--- a/include/linux/blk_types.h
+++ b/include/linux/blk_types.h
@@ -59,12 +59,6 @@ struct bio {
         unsigned int            bi_seg_front_size;
         unsigned int            bi_seg_back_size;
 
-        unsigned int            bi_max_vecs;    /* max bvl_vecs we can hold */
-
-        atomic_t                bi_cnt;         /* pin count */
-
-        struct bio_vec          *bi_io_vec;     /* the actual vec list */
-
         bio_end_io_t            *bi_end_io;
 
         void                    *bi_private;
@@ -80,6 +74,16 @@ struct bio {
         struct bio_integrity_payload *bi_integrity;  /* data integrity */
 #endif
 
+        /*
+         * Everything starting with bi_max_vecs will be preserved by bio_reset()
+         */
+
+        unsigned int            bi_max_vecs;    /* max bvl_vecs we can hold */
+
+        atomic_t                bi_cnt;         /* pin count */
+
+        struct bio_vec          *bi_io_vec;     /* the actual vec list */
+
         /* If bi_pool is non NULL, bi_destructor is not called */
         struct bio_set          *bi_pool;
 
@@ -93,6 +97,8 @@ struct bio {
         struct bio_vec          bi_inline_vecs[0];
 };
 
+#define BIO_RESET_BYTES         offsetof(struct bio, bi_max_vecs)
+
 /*
  * bio flags
  */
@@ -108,6 +114,13 @@ struct bio {
 #define BIO_FS_INTEGRITY 9      /* fs owns integrity data, not block layer */
 #define BIO_QUIET       10      /* Make BIO Quiet */
 #define BIO_MAPPED_INTEGRITY 11/* integrity metadata has been remapped */
+
+/*
+ * Flags starting here get preserved by bio_reset() - this includes
+ * BIO_POOL_IDX()
+ */
+#define BIO_RESET_BITS  12
+
 #define bio_flagged(bio, flag)  ((bio)->bi_flags & (1 << (flag)))
 
 /*