dm snapshot: add new persistent store option to support overflow

This change “dm snapshot: add new persistent store option to support overflow” in Linux kernel is authored by Mike Snitzer <snitzer [at] redhat.com> on Thu Oct 8 18:05:41 2015 -0400.

dm snapshot: add new persistent store option to support overflow

Commit 76c44f6d80 introduced the possibly for "Overflow" to be reported
by the snapshot device's status.  Older userspace (e.g. lvm2) does not
handle the "Overflow" status response.

Fix this incompatibility by requiring newer userspace code, that can
cope with "Overflow", request the persistent store with overflow support
by using "PO" (Persistent with Overflow) for the snapshot store type.

Reported-by: Zdenek Kabelac <zkabelac@redhat.com>
Fixes: 76c44f6d80 ("dm snapshot: don't invalidate on-disk image on snapshot write overflow")
Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

This Linux change may have been applied to various maintained Linux releases and you can find Linux releases including commit b0d3cc0.

There are 55 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 Documentation/device-mapper/snapshot.txt | 10 +++++++---
 drivers/md/dm-exception-store.c          |  6 +++---
 drivers/md/dm-exception-store.h          |  5 +++--
 drivers/md/dm-snap-persistent.c          | 17 ++++++++++++++---
 drivers/md/dm-snap-transient.c           |  3 +--
 drivers/md/dm-snap.c                     | 14 +++++++++-----
 6 files changed, 37 insertions(+), 18 deletions(-)

diff --git a/Documentation/device-mapper/snapshot.txt b/Documentation/device-mapper/snapshot.txt
index 0d5bc46..ad6949b 100644
--- a/Documentation/device-mapper/snapshot.txt
+++ b/Documentation/device-mapper/snapshot.txt
@@ -41,9 +41,13 @@ useless and be disabled, returning errors.  So it is important to monitor
 the amount of free space and expand the <COW device> before it fills up.
 
 <persistent?> is P (Persistent) or N (Not persistent - will not survive
-after reboot).
-The difference is that for transient snapshots less metadata must be
-saved on disk - they can be kept in memory by the kernel.
+after reboot).  O (Overflow) can be added as a persistent store option
+to allow userspace to advertise its support for seeing "Overflow" in the
+snapshot status.  So supported store types are "P", "PO" and "N".
+
+The difference between persistent and transient is with transient
+snapshots less metadata must be saved on disk - they can be kept in
+memory by the kernel.
 
 
 * snapshot-merge <origin> <COW device> <persistent> <chunksize>
diff --git a/drivers/md/dm-exception-store.c b/drivers/md/dm-exception-store.c
index ebaa4f8..192bb8b 100644
--- a/drivers/md/dm-exception-store.c
+++ b/drivers/md/dm-exception-store.c
@@ -203,7 +203,7 @@ int dm_exception_store_create(struct dm_target *ti, int argc, char **argv,
 		return -EINVAL;
 	}
 
-	tmp_store = kmalloc(sizeof(*tmp_store), GFP_KERNEL);
+	tmp_store = kzalloc(sizeof(*tmp_store), GFP_KERNEL);
 	if (!tmp_store) {
 		ti->error = "Exception store allocation failed";
 		return -ENOMEM;
@@ -215,7 +215,7 @@ int dm_exception_store_create(struct dm_target *ti, int argc, char **argv,
 	else if (persistent == 'N')
 		type = get_type("N");
 	else {
-		ti->error = "Persistent flag is not P or N";
+		ti->error = "Exception store type is not P or N";
 		r = -EINVAL;
 		goto bad_type;
 	}
@@ -233,7 +233,7 @@ int dm_exception_store_create(struct dm_target *ti, int argc, char **argv,
 	if (r)
 		goto bad;
 
-	r = type->ctr(tmp_store, 0, NULL);
+	r = type->ctr(tmp_store, (strlen(argv[0]) > 1 ? &argv[0][1] : NULL));
 	if (r) {
 		ti->error = "Exception store type constructor failed";
 		goto bad;
diff --git a/drivers/md/dm-exception-store.h b/drivers/md/dm-exception-store.h
index 0b25362..fae34e7 100644
--- a/drivers/md/dm-exception-store.h
+++ b/drivers/md/dm-exception-store.h
@@ -42,8 +42,7 @@ struct dm_exception_store_type {
 	const char *name;
 	struct module *module;
 
-	int (*ctr) (struct dm_exception_store *store,
-		    unsigned argc, char **argv);
+	int (*ctr) (struct dm_exception_store *store, char *options);
 
 	/*
 	 * Destroys this object when you've finished with it.
@@ -123,6 +122,8 @@ struct dm_exception_store {
 	unsigned chunk_shift;
 
 	void *context;
+
+	bool userspace_supports_overflow;
 };
 
 /*
diff --git a/drivers/md/dm-snap-persistent.c b/drivers/md/dm-snap-persistent.c
index bf71583..aeacad9 100644
--- a/drivers/md/dm-snap-persistent.c
+++ b/drivers/md/dm-snap-persistent.c
@@ -7,6 +7,7 @@
 
 #include "dm-exception-store.h"
 
+#include <linux/ctype.h>
 #include <linux/mm.h>
 #include <linux/pagemap.h>
 #include <linux/vmalloc.h>
@@ -843,8 +844,7 @@ static void persistent_drop_snapshot(struct dm_exception_store *store)
 		DMWARN("write header failed");
 }
 
-static int persistent_ctr(struct dm_exception_store *store,
-			  unsigned argc, char **argv)
+static int persistent_ctr(struct dm_exception_store *store, char *options)
 {
 	struct pstore *ps;
 
@@ -873,6 +873,16 @@ static int persistent_ctr(struct dm_exception_store *store,
 		return -ENOMEM;
 	}
 
+	if (options) {
+		char overflow = toupper(options[0]);
+		if (overflow == 'O')
+			store->userspace_supports_overflow = true;
+		else {
+			DMERR("Unsupported persistent store option: %s", options);
+			return -EINVAL;
+		}
+	}
+
 	store->context = ps;
 
 	return 0;
@@ -888,7 +898,8 @@ static unsigned persistent_status(struct dm_exception_store *store,
 	case STATUSTYPE_INFO:
 		break;
 	case STATUSTYPE_TABLE:
-		DMEMIT(" P %llu", (unsigned long long)store->chunk_size);
+		DMEMIT(" %s %llu", store->userspace_supports_overflow ? "PO" : "P",
+		       (unsigned long long)store->chunk_size);
 	}
 
 	return sz;
diff --git a/drivers/md/dm-snap-transient.c b/drivers/md/dm-snap-transient.c
index 1ce9a25..9b7c8c8 100644
--- a/drivers/md/dm-snap-transient.c
+++ b/drivers/md/dm-snap-transient.c
@@ -70,8 +70,7 @@ static void transient_usage(struct dm_exception_store *store,
 	*metadata_sectors = 0;
 }
 
-static int transient_ctr(struct dm_exception_store *store,
-			 unsigned argc, char **argv)
+static int transient_ctr(struct dm_exception_store *store, char *options)
 {
 	struct transient_c *tc;
 
diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index c0bcd65..c06b74e 100644
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -1098,7 +1098,7 @@ static void stop_merge(struct dm_snapshot *s)
 }
 
 /*
- * Construct a snapshot mapping: <origin_dev> <COW-dev> <p/n> <chunk-size>
+ * Construct a snapshot mapping: <origin_dev> <COW-dev> <p|po|n> <chunk-size>
  */
 static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 {
@@ -1302,6 +1302,7 @@ static void __handover_exceptions(struct dm_snapshot *snap_src,
 
 	u.store_swap = snap_dest->store;
 	snap_dest->store = snap_src->store;
+	snap_dest->store->userspace_supports_overflow = u.store_swap->userspace_supports_overflow;
 	snap_src->store = u.store_swap;
 
 	snap_dest->store->snap = snap_dest;
@@ -1739,8 +1740,11 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio)
 
 			pe = __find_pending_exception(s, pe, chunk);
 			if (!pe) {
-				s->snapshot_overflowed = 1;
-				DMERR("Snapshot overflowed: Unable to allocate exception.");
+				if (s->store->userspace_supports_overflow) {
+					s->snapshot_overflowed = 1;
+					DMERR("Snapshot overflowed: Unable to allocate exception.");
+				} else
+					__invalidate_snapshot(s, -ENOMEM);
 				r = -EIO;
 				goto out_unlock;
 			}
@@ -2365,7 +2369,7 @@ static int origin_iterate_devices(struct dm_target *ti,
 
 static struct target_type snapshot_target = {
 	.name    = "snapshot",
-	.version = {1, 14, 0},
+	.version = {1, 15, 0},
 	.module  = THIS_MODULE,
 	.ctr     = snapshot_ctr,
 	.dtr     = snapshot_dtr,
@@ -2379,7 +2383,7 @@ static int origin_iterate_devices(struct dm_target *ti,
 
 static struct target_type merge_target = {
 	.name    = dm_snapshot_merge_target_name,
-	.version = {1, 3, 0},
+	.version = {1, 4, 0},
 	.module  = THIS_MODULE,
 	.ctr     = snapshot_ctr,
 	.dtr     = snapshot_dtr,

The commit for this change in Linux stable tree is b0d3cc0 (patch).

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