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>

1 files changed, 24 insertions, 0 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