[RFC PATCH v1 05/17] block: add bio_alloc_atomic() for atomic bio users

[Yu Kuai <yukuai@kernel.org>]

From: Yu Kuai <yukuai@fygo.io>

Add bio_alloc_atomic() for callers that need a GFP_ATOMIC bio from the
default bio set but cannot safely pass a bdev during allocation. The
helper returns an unattached bio, leaving callers to set bi_bdev and
attach blkcg state explicitly before submission.

Use the helper for virtio-pmem flush child bios and OCFS2 heartbeat I/O.
Both allocate bios from atomic paths and must avoid creating missing blkgs
once blkg creation is protected by q->blkcg_mutex. virtio-pmem clones the
parent bio's blkg association; OCFS2 binds heartbeat I/O to the root blkg.

Signed-off-by: Yu Kuai <yukuai@fygo.io>
---
 drivers/nvdimm/nd_virtio.c   |  8 ++++----
 fs/ocfs2/cluster/heartbeat.c | 15 ++++++++++++---
 include/linux/bio.h          |  6 ++++++
 3 files changed, 22 insertions(+), 7 deletions(-)

diff --git a/drivers/nvdimm/nd_virtio.c b/drivers/nvdimm/nd_virtio.c
index 4176046627be..13d1ed1c466c 100644
--- a/drivers/nvdimm/nd_virtio.c
+++ b/drivers/nvdimm/nd_virtio.c
@@ -115,13 +115,13 @@ int async_pmem_flush(struct nd_region *nd_region, struct bio *bio)
 	 * parent bio. Otherwise directly call nd_region flush.
 	 */
 	if (bio && bio->bi_iter.bi_sector != -1) {
-		struct bio *child = bio_alloc(bio->bi_bdev, 0,
-					      REQ_OP_WRITE | REQ_PREFLUSH,
-					      GFP_ATOMIC);
+		struct bio *child = bio_alloc_atomic(0,
+						REQ_OP_WRITE | REQ_PREFLUSH);
 
 		if (!child)
 			return -ENOMEM;
-		bio_clone_blkg_association(child, bio);
+		child->bi_bdev = bio->bi_bdev;
+			bio_clone_blkg_association(child, bio);
 		child->bi_iter.bi_sector = -1;
 		bio_chain(child, bio);
 		submit_bio(child);
diff --git a/fs/ocfs2/cluster/heartbeat.c b/fs/ocfs2/cluster/heartbeat.c
index d12784aaaa4b..ec70f3b62837 100644
--- a/fs/ocfs2/cluster/heartbeat.c
+++ b/fs/ocfs2/cluster/heartbeat.c
@@ -10,6 +10,7 @@
 #include <linux/module.h>
 #include <linux/fs.h>
 #include <linux/bio.h>
+#include <linux/blk-cgroup.h>
 #include <linux/blkdev.h>
 #include <linux/delay.h>
 #include <linux/file.h>
@@ -519,16 +520,24 @@ static struct bio *o2hb_setup_one_bio(struct o2hb_region *reg,
 	struct bio *bio;
 	struct page *page;
 
-	/* Testing has shown this allocation to take long enough under
+	/*
+	 * Testing has shown this allocation to take long enough under
 	 * GFP_KERNEL that the local node can get fenced. It would be
 	 * nicest if we could pre-allocate these bios and avoid this
-	 * all together. */
-	bio = bio_alloc(reg_bdev(reg), 16, opf, GFP_ATOMIC);
+	 * all together.
+	 *
+	 * Use the atomic bio allocation helper so bio_init() does not create a
+	 * missing blkg. Heartbeat IO is cluster-liveness IO, so account it to
+	 * the root blkcg instead.
+	 */
+	bio = bio_alloc_atomic(16, opf);
 	if (!bio) {
 		mlog(ML_ERROR, "Could not alloc slots BIO!\n");
 		bio = ERR_PTR(-ENOMEM);
 		goto bail;
 	}
+	bio->bi_bdev = reg_bdev(reg);
+	bio_associate_blkg_from_css(bio, blkcg_root_css);
 
 	/* Must put everything in 512 byte sectors for the bio... */
 	bio->bi_iter.bi_sector = (reg->hr_start_block + cs) << (bits - 9);
diff --git a/include/linux/bio.h b/include/linux/bio.h
index 8f33f717b14f..f7d94d37893f 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -366,6 +366,12 @@ static inline struct bio *bio_alloc(struct block_device *bdev,
 	return bio_alloc_bioset(bdev, nr_vecs, opf, gfp_mask, &fs_bio_set);
 }
 
+static inline struct bio *bio_alloc_atomic(unsigned short nr_vecs,
+					   blk_opf_t opf)
+{
+	return bio_alloc_bioset(NULL, nr_vecs, opf, GFP_ATOMIC, &fs_bio_set);
+}
+
 void submit_bio(struct bio *bio);
 
 extern void bio_endio(struct bio *);
-- 
2.51.0