block: Improvements to bounce-buffer handling

Since commit 23688bf4f830 ("block: ensure to split after potentially
bouncing a bio") blk_queue_bounce() is called *before*
blk_queue_split().
This means that:
 1/ the comments blk_queue_split() about bounce buffers are
    irrelevant, and
 2/ a very large bio (more than BIO_MAX_PAGES) will no longer be
    split before it arrives at blk_queue_bounce(), leading to the
    possibility that bio_clone_bioset() will fail and a NULL
    will be dereferenced.

Separately, blk_queue_bounce() shouldn't use fs_bio_set as the bio
being copied could be from the same set, and this could lead to a
deadlock.

So:
 - allocate 2 private biosets for blk_queue_bounce, one for
   splitting enormous bios and one for cloning bios.
 - add code to split a bio that exceeds BIO_MAX_PAGES.
 - Fix up the comments in blk_queue_split()

Credit-to: Ming Lei <tom.leiming@gmail.com> (suggested using single bio_for_each_segment loop)
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 4 insertions, 10 deletions

diff --git a/block/blk-merge.c b/block/blk-merge.c
index d59074556703..51c84540d3bb 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -117,17 +117,11 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
                  * each holds at most BIO_MAX_PAGES bvecs because
                  * bio_clone() can fail to allocate big bvecs.
                  *
-                 * It should have been better to apply the limit per
-                 * request queue in which bio_clone() is involved,
-                 * instead of globally. The biggest blocker is the
-                 * bio_clone() in bio bounce.
+                 * Those drivers which will need to use bio_clone()
+                 * should tell us in some way.  For now, impose the
+                 * BIO_MAX_PAGES limit on all queues.
                  *
-                 * If bio is splitted by this reason, we should have
-                 * allowed to continue bios merging, but don't do
-                 * that now for making the change simple.
-                 *
-                 * TODO: deal with bio bounce's bio_clone() gracefully
-                 * and convert the global limit into per-queue limit.
+                 * TODO: handle users of bio_clone() differently.
                  */
                 if (bvecs++ >= BIO_MAX_PAGES)
                         goto split;