dm-qcow2: Introduce driver to create block devices over QCOW2 files

Driver for attaching QCOW2 files as block devices. It cares
about performance-critical actions like actual IO and
snapshot COW and merge, while complex and fast metadata-related
service actions (e.g., snapshot creation and resize)
are delegated to userspace.

Suspend/resume is a barrier between the driver and userspace.
On suspend the driver brings QCOW2 images in consistent state,
while on resume it re-reads metadata from QCOW2 images.
Userspace proceeds metadata-related service actions while
device is suspended, and it never does this on running device.
This demarcation allows driver code to be pretty small.

Implemented full support for QCOW2 format: compressed clusters,
internal snapshots, backing files and extended L2 table.
Current limitations: the lowest backing file must be QCOW2
(RAW backing files are not supported), cluster and subcluster
sizes for all backing files in chain must be equal (I'm going
to change this in the future).

Also, implemented backward merge (from top image to its backing file).

The driver is request based, since this allows to use blk-mq
merging of request. Driver splits requests itself, and every
request (i.e., qio) after splitting fits a single cluster.
(In some cases it is worth to create bigger splits, and this
is a subject of further optimizations).

Signed-off-by: Kirill Tkhai <kirill.tkhai@openvz.org>

drivers/md/Kconfig

@@ -653,4 +653,21 @@ config DM_AUDIT
 	  Enables audit logging of several security relevant events in the
 	  particular device-mapper targets, especially the integrity target.
 
+config DM_QCOW2
+	tristate "QCOW2 target support"
+	depends on BLK_DEV_DM
+	depends on ZLIB_INFLATE
+	help
+	  Driver for attaching QCOW2 files as block devices. It cares
+	  about performance-critical actions like actual IO and
+	  snapshot COW and merge, while complex and fast metadata-related
+	  service actions (e.g., snapshot creation and resize)
+	  are delegated to userspace.
+
+	  Suspend/resume is a barrier between the driver and userspace.
+	  On suspend the driver brings QCOW2 images in consistent state,
+	  while on resume it re-reads metadata from QCOW2 images. Thus,
+	  userspace proceeds metadata-related service actions, while
+	  device is suspended.
+
 endif # MD

drivers/md/Makefile

@@ -26,6 +26,7 @@ dm-era-y	+= dm-era-target.o
 dm-clone-y	+= dm-clone-target.o dm-clone-metadata.o
 dm-verity-y	+= dm-verity-target.o
 dm-zoned-y	+= dm-zoned-target.o dm-zoned-metadata.o dm-zoned-reclaim.o
+dm-qcow2-y	+= dm-qcow2-target.o dm-qcow2-map.o dm-qcow2-cmd.o
 
 md-mod-y	+= md.o md-bitmap.o
 raid456-y	+= raid5.o raid5-cache.o raid5-ppl.o
@@ -83,6 +84,7 @@ obj-$(CONFIG_DM_LOG_WRITES)	+= dm-log-writes.o
 obj-$(CONFIG_DM_INTEGRITY)	+= dm-integrity.o
 obj-$(CONFIG_DM_ZONED)		+= dm-zoned.o
 obj-$(CONFIG_DM_WRITECACHE)	+= dm-writecache.o
+obj-$(CONFIG_DM_QCOW2)		+= dm-qcow2.o
 
 ifeq ($(CONFIG_DM_INIT),y)
 dm-mod-objs			+= dm-init.o