Fix zfs_rename to update symlinks under /dev (Fixes #408)

[schakrava]

When a zvol consisting of snapshots is renamed, the symlinks under
/dev for snapshots are not renamed at all. Exporting and reimporting
the zpool updates the link properly but that is not desirable to
do. We need to update udev in zfs_rename to update these symlinks as
part of zfs rename.

I've update the zfs_rename function to remove symlinks prior to
renaming the volume and create new links upon success or restore old
links upon failure. Also, I've moved parentname and zhrp variable
asignments up towards the beginning as they are needed for both
recursive and volume rename cases. I've also added supporting
functions zfs_create_link_snapvol and zfs_remove_link_snapvol to be
used as callbacks for zfs_iter_snapshots

[behlendorf]

Thanks for working on this. After reviewing your proposed patch I have one big outstanding question. Does this bug exist on OpenSolaris as well? After reviewing all of the relevant code I don't see how the OpenSolaris version handles this case either so it seems like it must. If you have access to an OpenSolaris box it would be interesting to test.

Assuming that the Solaris system doesn't handle this case either and we're both not just over looking the existing machinery let me suggest an alternate fix. The reason this works for the volume itself and not the snapshot can be found in zfs_ioc_rename(). Notice the zvol_remove_minor(); zvol_create_minor() block. This has the effect of unregistering and registering a new miror with sysfs which will result in udev automatically updating the symlinks. It should be pretty straight forward to extend this same basic mechanism to cover snapshots of the volume.

Here's some preliminary non-functional code to outline what I have in mind. You'll need to fix up the dataset names and such but it should do what you want by iterating over the snapshots allowing you to remove and recreate them.

diff --git a/include/sys/zvol.h b/include/sys/zvol.h
index 815b186..df3fbde 100644
--- a/include/sys/zvol.h
+++ b/include/sys/zvol.h
@@ -41,6 +41,7 @@ extern int zvol_create_minor(const char *);
 extern int zvol_create_minors(const char *);
 extern int zvol_remove_minor(const char *);
 extern void zvol_remove_minors(const char *);
+extern int zvol_rename_minor(char *, char *);
 extern int zvol_set_volsize(const char *, uint64_t);
 extern int zvol_set_volblocksize(const char *, uint64_t);
 
diff --git a/module/zfs/zfs_ioctl.c b/module/zfs/zfs_ioctl.c
index c8ff035..a47eaf9 100644
--- a/module/zfs/zfs_ioctl.c
+++ b/module/zfs/zfs_ioctl.c
@@ -3300,10 +3300,8 @@ zfs_ioc_rename(zfs_cmd_t *zc)
        }
 
        err = dmu_objset_rename(zc->zc_name, zc->zc_value, recursive);
-       if ((err == 0) && (zc->zc_objset_type == DMU_OST_ZVOL)) {
-               (void) zvol_remove_minor(zc->zc_name);
-               (void) zvol_create_minor(zc->zc_value);
-       }
+       if ((err == 0) && (zc->zc_objset_type == DMU_OST_ZVOL))
+               (void) zvol_rename_minor(zc->zc_name, zc->zc_value);
 
        return (err);
 }
diff --git a/module/zfs/zvol.c b/module/zfs/zvol.c
index a25fbbe..b2a233e 100644
--- a/module/zfs/zvol.c
+++ b/module/zfs/zvol.c
@@ -768,6 +768,34 @@ zvol_remove(zvol_state_t *zv_remove)
 }
 
 static int
+zvol_rename_cb(const char *dsname, void *arg)
+{
+       char *new_name = arg;
+       int error = 0;
+
+       printk("dsname = %s, new_name = %s\n", dsname, new_name);
+
+       (void) zvol_remove_minor(dsname);
+       (void) zvol_create_minor(new_name);
+
+       return (error);
+}
+
+/*
+ * Rename a zvol removing as creating new minors as needed.  This
+ * is required to ensure sysfs is notified and udev triggered.
+ */
+int
+zvol_rename_minor(char *old_name, char *new_name)
+{
+       int error = 0;
+
+       error = dmu_objset_find(old_name, zvol_rename_cb,
+           new_name, DS_FIND_SNAPSHOTS);
+
+       return (error);
+}
+
+static int
 zvol_first_open(zvol_state_t *zv)
 {
        objset_t *os;

[behlendorf]

Fixed by commit 0e20a31 which was based on this patch originally.