block: Improvements to bounce-buffer handling

Since commit 23688bf4f8 ("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>

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;

block/bounce.c

@@ -26,6 +26,7 @@
 #define POOL_SIZE	64
 #define ISA_POOL_SIZE	16
 
+struct bio_set *bounce_bio_set, *bounce_bio_split;
 static mempool_t *page_pool, *isa_page_pool;
 
 #if defined(CONFIG_HIGHMEM) || defined(CONFIG_NEED_BOUNCE_POOL)
@@ -40,6 +41,14 @@ static __init int init_emergency_pool(void)
 	BUG_ON(!page_pool);
 	pr_info("pool size: %d pages\n", POOL_SIZE);
 
+	bounce_bio_set = bioset_create(BIO_POOL_SIZE, 0, BIOSET_NEED_BVECS);
+	BUG_ON(!bounce_bio_set);
+	if (bioset_integrity_create(bounce_bio_set, BIO_POOL_SIZE))
+		BUG_ON(1);
+
+	bounce_bio_split = bioset_create(BIO_POOL_SIZE, 0, 0);
+	BUG_ON(!bounce_bio_split);
+
 	return 0;
 }
 
@@ -186,15 +195,26 @@ static void __blk_queue_bounce(struct request_queue *q, struct bio **bio_orig,
 	int rw = bio_data_dir(*bio_orig);
 	struct bio_vec *to, from;
 	struct bvec_iter iter;
-	unsigned i;
+	unsigned i = 0;
+	bool bounce = false;
+	int sectors = 0;
 
-	bio_for_each_segment(from, *bio_orig, iter)
+	bio_for_each_segment(from, *bio_orig, iter) {
+		if (i++ < BIO_MAX_PAGES)
+			sectors += from.bv_len >> 9;
 		if (page_to_pfn(from.bv_page) > queue_bounce_pfn(q))
-			goto bounce;
+			bounce = true;
+	}
+	if (!bounce)
+		return;
 
-	return;
-bounce:
-	bio = bio_clone_bioset(*bio_orig, GFP_NOIO, fs_bio_set);
+	if (sectors < bio_sectors(*bio_orig)) {
+		bio = bio_split(*bio_orig, sectors, GFP_NOIO, bounce_bio_split);
+		bio_chain(bio, *bio_orig);
+		generic_make_request(*bio_orig);
+		*bio_orig = bio;
+	}
+	bio = bio_clone_bioset(*bio_orig, GFP_NOIO, bounce_bio_set);
 
 	bio_for_each_segment_all(to, bio, i) {
 		struct page *page = to->bv_page;