[PATCH v5 3/6] block: add blk_interposer

[Sergei Shtepa <sergei.shtepa@veeam.com>]

blk_interposer allows to intercept bio requests, remap bio
to another devices or add new bios.

Signed-off-by: Sergei Shtepa <sergei.shtepa@veeam.com>
---
 block/bio.c               |  2 +
 block/blk-core.c          | 35 ++++++++++++++++
 block/genhd.c             | 86 +++++++++++++++++++++++++++++++++++++++
 include/linux/blk_types.h |  6 ++-
 include/linux/genhd.h     | 18 ++++++++
 5 files changed, 145 insertions(+), 2 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index 1f2cc1fbe283..f6f135eb84b5 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -684,6 +684,8 @@ void __bio_clone_fast(struct bio *bio, struct bio *bio_src)
 	bio_set_flag(bio, BIO_CLONED);
 	if (bio_flagged(bio_src, BIO_THROTTLED))
 		bio_set_flag(bio, BIO_THROTTLED);
+	if (bio_flagged(bio_src, BIO_INTERPOSED))
+		bio_set_flag(bio, BIO_INTERPOSED);
 	bio->bi_opf = bio_src->bi_opf;
 	bio->bi_ioprio = bio_src->bi_ioprio;
 	bio->bi_write_hint = bio_src->bi_write_hint;
diff --git a/block/blk-core.c b/block/blk-core.c
index 7663a9b94b80..e830eb5ae7f0 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1032,6 +1032,34 @@ static blk_qc_t __submit_bio_noacct_mq(struct bio *bio)
 	return ret;
 }
 
+static blk_qc_t __submit_bio_interposed(struct bio *bio)
+{
+	struct bio_list bio_list[2] = { };
+	blk_qc_t ret = BLK_QC_T_NONE;
+
+	current->bio_list = bio_list;
+	if (likely(bio_queue_enter(bio) == 0)) {
+		struct gendisk *disk = bio->bi_disk;
+
+		if (likely(blk_has_interposer(disk))) {
+			bio_set_flag(bio, BIO_INTERPOSED);
+			disk->interposer->ip_submit_bio(bio);
+		} else {
+			/* interposer was removed */
+			bio_list_add(&current->bio_list[0], bio);
+		}
+
+		blk_queue_exit(disk->queue);
+	}
+	current->bio_list = NULL;
+
+	/* Resubmit remaining bios */
+	while ((bio = bio_list_pop(&bio_list[0])))
+		ret = submit_bio_noacct(bio);
+
+	return ret;
+}
+
 /**
  * submit_bio_noacct - re-submit a bio to the block device layer for I/O
  * @bio:  The bio describing the location in memory and on the device.
@@ -1057,6 +1085,13 @@ blk_qc_t submit_bio_noacct(struct bio *bio)
 		return BLK_QC_T_NONE;
 	}
 
+	/*
+	 * Checking the BIO_INTERPOSED flag is necessary so that the bio
+	 * created by the blk_interposer do not get to it for processing.
+	 */
+	if (blk_has_interposer(bio->bi_disk) &&
+	    !bio_flagged(bio, BIO_INTERPOSED))
+		return __submit_bio_interposed(bio);
 	if (!bio->bi_disk->fops->submit_bio)
 		return __submit_bio_noacct_mq(bio);
 	return __submit_bio_noacct(bio);
diff --git a/block/genhd.c b/block/genhd.c
index 9e741a4f351b..728f1e68bb2d 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -30,6 +30,11 @@
 static struct kobject *block_depr;
 
 DECLARE_RWSEM(bdev_lookup_sem);
+/*
+ * Prevents different block-layer interposers from attaching or detaching
+ * to the disk at the same time.
+ */
+DEFINE_MUTEX(blk_interposer_attach_lock);
 
 /* for extended dynamic devt allocation, currently only one major is used */
 #define NR_EXT_DEVT		(1 << MINORBITS)
@@ -2149,3 +2154,84 @@ static void disk_release_events(struct gendisk *disk)
 	WARN_ON_ONCE(disk->ev && disk->ev->block != 1);
 	kfree(disk->ev);
 }
+
+/**
+ * blk_interposer_attach - Attach interposer to disk
+ * @disk: target disk
+ * @interposer: block device interposer
+ * @ip_submit_bio: hook for submit_bio()
+ *
+ * Returns:
+ *     -EINVAL if @interposer is NULL.
+ *     -EPERM if queue is not frozen.
+ *     -EBUSY if the block device already has @interposer.
+ *     -EALREADY if the block device already has @interposer with same callback.
+ *
+ * Disk must be frozen by blk_mq_freeze_queue().
+ */
+int blk_interposer_attach(struct gendisk *disk, struct blk_interposer *interposer,
+			  const ip_submit_bio_t ip_submit_bio)
+{
+	int ret = 0;
+
+	if (WARN_ON(!interposer))
+		return -EINVAL;
+
+	if (!blk_mq_is_queue_frozen(disk->queue))
+		return -EPERM;
+
+	mutex_lock(&blk_interposer_attach_lock);
+	if (blk_has_interposer(disk)) {
+		if (disk->interposer->ip_submit_bio == ip_submit_bio)
+			ret = -EALREADY;
+		else
+			ret = -EBUSY;
+		goto out;
+	}
+
+	interposer->ip_submit_bio = ip_submit_bio;
+	interposer->disk = disk;
+
+	disk->interposer = interposer;
+out:
+	mutex_unlock(&blk_interposer_attach_lock);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(blk_interposer_attach);
+
+/**
+ * blk_interposer_detach - Detach interposer from disk
+ * @interposer: block device interposer
+ * @ip_submit_bio: hook for submit_bio()
+ *
+ * Disk must be frozen by blk_mq_freeze_queue().
+ */
+void blk_interposer_detach(struct blk_interposer *interposer,
+			  const ip_submit_bio_t ip_submit_bio)
+{
+	struct gendisk *disk;
+
+	if (WARN_ON(!interposer))
+		return;
+
+	mutex_lock(&blk_interposer_attach_lock);
+
+	/* Check if the interposer is still active. */
+	disk = interposer->disk;
+	if (WARN_ON(!disk))
+		goto out;
+
+	if (WARN_ON(!blk_mq_is_queue_frozen(disk->queue)))
+		goto out;
+
+	/* Check if it is really our interposer. */
+	if (WARN_ON(disk->interposer->ip_submit_bio != ip_submit_bio))
+		goto out;
+
+	disk->interposer = NULL;
+	interposer->disk = NULL;
+out:
+	mutex_unlock(&blk_interposer_attach_lock);
+}
+EXPORT_SYMBOL_GPL(blk_interposer_detach);
diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
index 866f74261b3b..6c1351d7b73f 100644
--- a/include/linux/blk_types.h
+++ b/include/linux/blk_types.h
@@ -227,7 +227,7 @@ struct bio {
 						 * top bits REQ_OP. Use
 						 * accessors.
 						 */
-	unsigned short		bi_flags;	/* status, etc and bvec pool number */
+	unsigned int		bi_flags;	/* status, etc and bvec pool number */
 	unsigned short		bi_ioprio;
 	unsigned short		bi_write_hint;
 	blk_status_t		bi_status;
@@ -304,6 +304,8 @@ enum {
 				 * of this bio. */
 	BIO_CGROUP_ACCT,	/* has been accounted to a cgroup */
 	BIO_TRACKED,		/* set if bio goes through the rq_qos path */
+	BIO_INTERPOSED,		/* bio has been interposed and can be moved to
+				 * a different disk */
 	BIO_FLAG_LAST
 };
 
@@ -322,7 +324,7 @@ enum {
  * freed.
  */
 #define BVEC_POOL_BITS		(3)
-#define BVEC_POOL_OFFSET	(16 - BVEC_POOL_BITS)
+#define BVEC_POOL_OFFSET	(32 - BVEC_POOL_BITS)
 #define BVEC_POOL_IDX(bio)	((bio)->bi_flags >> BVEC_POOL_OFFSET)
 #if (1<< BVEC_POOL_BITS) < (BVEC_POOL_NR+1)
 # error "BVEC_POOL_BITS is too small"
diff --git a/include/linux/genhd.h b/include/linux/genhd.h
index 809aaa32d53c..f68c8e83b4f1 100644
--- a/include/linux/genhd.h
+++ b/include/linux/genhd.h
@@ -134,6 +134,13 @@ struct blk_integrity {
 	unsigned char				tag_size;
 };
 
+typedef void (*ip_submit_bio_t) (struct bio *bio);
+
+struct blk_interposer {
+	ip_submit_bio_t ip_submit_bio;
+	struct gendisk *disk;
+};
+
 struct gendisk {
 	/* major, first_minor and minors are input parameters only,
 	 * don't use directly.  Use disk_devt() and disk_max_parts().
@@ -158,6 +165,7 @@ struct gendisk {
 
 	const struct block_device_operations *fops;
 	struct request_queue *queue;
+	struct blk_interposer *interposer;
 	void *private_data;
 
 	int flags;
@@ -346,4 +354,14 @@ static inline void printk_all_partitions(void)
 }
 #endif /* CONFIG_BLOCK */
 
+/*
+ * block layer interposer
+ */
+#define blk_has_interposer(d) ((d)->interposer != NULL)
+
+int blk_interposer_attach(struct gendisk *disk, struct blk_interposer *interposer,
+			  const ip_submit_bio_t ip_submit_bio);
+void blk_interposer_detach(struct blk_interposer *interposer,
+			   const ip_submit_bio_t ip_submit_bio);
+
 #endif /* _LINUX_GENHD_H */
-- 
2.20.1