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Commit b0d3cc01 authored by Mike Snitzer's avatar Mike Snitzer
Browse files

dm snapshot: add new persistent store option to support overflow 



Commit 76c44f6d 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: default avatarZdenek Kabelac <zkabelac@redhat.com>
Fixes: 76c44f6d ("dm snapshot: don't invalidate on-disk image on snapshot write overflow")
Reviewed-by: default avatarMikulas Patocka <mpatocka@redhat.com>
Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
parent 2bffa150

Documentation/device-mapper/snapshot.txt

+7 −3
Original line number Diff line number Diff line
@@ -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>

drivers/md/dm-exception-store.c

+3 −3
Original line number Diff line number Diff line
@@ -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;

drivers/md/dm-exception-store.h

+3 −2
Original line number Diff line number Diff line
@@ -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;

};



/*

drivers/md/dm-snap-persistent.c

+14 −3
Original line number Diff line number Diff line
@@ -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;

drivers/md/dm-snap-transient.c

+1 −2
Original line number Diff line number Diff line
@@ -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;