Re: [PATCH v6 4/4] dm: add DM_INTERPOSED_FLAG

DM_INTERPOSED_FLAG allow to create dm targets on "the fly".
Underlying block device opens without a flag FMODE_EXCL.
Dm target receives bio from the original device via
blk_interposer.

Signed-off-by: Sergei Shtepa <redacted>
---
 drivers/md/dm-core.h          |   6 ++
 drivers/md/dm-ioctl.c         |   9 +++
 drivers/md/dm-table.c         | 115 +++++++++++++++++++++++++++++++---
 drivers/md/dm.c               |  38 +++++++----
 include/linux/device-mapper.h |   1 +
 include/uapi/linux/dm-ioctl.h |   6 ++
 6 files changed, 154 insertions(+), 21 deletions(-)

diff --git a/drivers/md/dm-core.h b/drivers/md/dm-core.h
index 5953ff2bd260..e5c845f9b1df 100644
--- a/drivers/md/dm-core.h
+++ b/drivers/md/dm-core.h

@@ -21,6 +21,8 @@
 
 #define DM_RESERVED_MAX_IOS		1024
 
+struct dm_interposed_dev;
+
 struct dm_kobject_holder {
 	struct kobject kobj;
 	struct completion completion;

@@ -114,6 +116,10 @@ struct mapped_device {
 	bool init_tio_pdu:1;
 
 	struct srcu_struct io_barrier;
+
+	/* for interposers logic */
+	bool is_interposed;
+	struct dm_interposed_dev *ip_dev;
 };
 
 void disable_discard(struct mapped_device *md);

diff --git a/drivers/md/dm-ioctl.c b/drivers/md/dm-ioctl.c
index 5e306bba4375..2bcb316144a1 100644
--- a/drivers/md/dm-ioctl.c
+++ b/drivers/md/dm-ioctl.c

@@ -1267,6 +1267,11 @@ static inline fmode_t get_mode(struct dm_ioctl *param)
 	return mode;
 }
 
+static inline bool get_interposer_flag(struct dm_ioctl *param)
+{
+	return (param->flags & DM_INTERPOSED_FLAG);
+}
+
 static int next_target(struct dm_target_spec *last, uint32_t next, void *end,
 		       struct dm_target_spec **spec, char **target_params)
 {

@@ -1338,6 +1343,8 @@ static int table_load(struct file *filp, struct dm_ioctl *param, size_t param_si
 	if (!md)
 		return -ENXIO;
 
+	md->is_interposed = get_interposer_flag(param);
+
 	r = dm_table_create(&t, get_mode(param), param->target_count, md);
 	if (r)
 		goto err;

@@ -2098,6 +2105,8 @@ int __init dm_early_create(struct dm_ioctl *dmi,
 	if (r)
 		goto err_hash_remove;
 
+	md->is_interposed = get_interposer_flag(dmi);
+
 	/* add targets */
 	for (i = 0; i < dmi->target_count; i++) {
 		r = dm_table_add_target(t, spec_array[i]->target_type,

diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index 95391f78b8d5..0b2f9b66ade5 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c

@@ -6,6 +6,7 @@
  */
 
 #include "dm-core.h"
+#include "dm-interposer.h"
 
 #include <linux/module.h>
 #include <linux/vmalloc.h>

@@ -225,12 +226,13 @@ void dm_table_destroy(struct dm_table *t)
 /*
  * See if we've already got a device in the list.
  */
-static struct dm_dev_internal *find_device(struct list_head *l, dev_t dev)
+static struct dm_dev_internal *find_device(struct list_head *l, dev_t dev, bool is_interposed)
 {
 	struct dm_dev_internal *dd;
 
 	list_for_each_entry (dd, l, list)
-		if (dd->dm_dev->bdev->bd_dev == dev)
+		if ((dd->dm_dev->bdev->bd_dev == dev) &&
+		    (dd->dm_dev->is_interposed == is_interposed))
 			return dd;
 
 	return NULL;

@@ -358,6 +360,90 @@ dev_t dm_get_dev_t(const char *path)
 }
 EXPORT_SYMBOL_GPL(dm_get_dev_t);
 
+/*
+ * Redirect bio from interposed device to dm device
+ */
+static void dm_interpose_fn(struct dm_interposed_dev *ip_dev, struct bio *bio)
+{
+	struct mapped_device *md = ip_dev->private;
+
+	if (bio_flagged(bio, BIO_REMAPPED)) {
+		/*
+		 * Since bio has already been remapped, we need to subtract
+		 * the block device offset from the beginning of the disk.
+		 */
+		bio->bi_iter.bi_sector -= get_start_sect(bio->bi_bdev);
+
+		bio_clear_flag(bio, BIO_REMAPPED);
+	}

+	if ((ofs + len) > bdev_nr_sectors(bdev)) {
+		DMERR("The specified range of sectors exceeds of the size of the block device.");
+		return -ERANGE;
+	}
+
+	md->ip_dev = kzalloc(sizeof(struct dm_interposed_dev), GFP_KERNEL);
+	if (!md->ip_dev)
+		return -ENOMEM;
+
+	if ((ofs == 0) && (len == 0))