mdadm: Patch to restrict --size when shrinking unless forced

["John Stoffel" <john@stoffel.org>]

Since Eli had such a horrible experience where he shrunk the
individual component raid device size, instead of growing the overall
raid by adding a device, I came up with this hacky patch to warn you
when you are about to shoot yourself in the foot.

The idea is it will warn you and exit unless you pass in the --force
(or -f) switch when using the command.  For example, on a set of loop
devices:

    # cat /proc/mdstat
    Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5]
    [raid4] [multipath] [faulty]
    md99 : active raid6 loop4p1[4] loop3p1[3] loop2p1[2] loop1p1[1]
    loop0p1[0]
	  606720 blocks super 1.2 level 6, 512k chunk, algorithm 2 [5/5]
	  [UUUUU]

    # ./mdadm --grow /dev/md99 --size 128
    mdadm: Cannot set device size smaller than current component_size of /dev/md99 array.  Use -f to force change.

    # ./mdadm --grow /dev/md99 --size 128 -f
    mdadm: component size of /dev/md99 has been set to 0K


I suspect I could do a better job of showing the original component
size, so that you have a chance of recovering even then.

But the patch:

diff --git a/Grow.c b/Grow.c
index 455c5f9..701590f 100755
--- a/Grow.c
+++ b/Grow.c
@@ -1561,7 +1561,7 @@ static int reshape_container(char *container, char *devname,
 			     char *backup_file, int verbose,
 			     int forked, int restart, int freeze_reshape);
 
-int Grow_reshape(char *devname, int fd,
+int Grow_reshape(char *devname, int fd, int force,
 		 struct mddev_dev *devlist,
 		 unsigned long long data_offset,
 		 struct context *c, struct shape *s)
@@ -1574,6 +1574,7 @@ int Grow_reshape(char *devname, int fd,
 	 * requested everything (if kernel supports freezing - 2.6.30).
 	 * The steps are:
 	 *  - change size (i.e. component_size)
+         *    - when shrinking, you must force the change 
 	 *  - change level
 	 *  - change layout/chunksize/ndisks
 	 *
@@ -1617,6 +1618,11 @@ int Grow_reshape(char *devname, int fd,
 		return 1;
 	}
 
+	if ((s->size < (unsigned)array.size) && !force) {
+	    pr_err("Cannot set device size smaller than current component_size of %s array.  Use -f to force change.\n",devname);
+	    return 1;
+	}
+
 	if (s->raiddisks && s->raiddisks < array.raid_disks && array.level > 1 &&
 	    get_linux_version() < 2006032 &&
 	    !check_env("MDADM_FORCE_FEWER")) {
diff --git a/ReadMe.c b/ReadMe.c
index 50d3807..46988ae 100644
--- a/ReadMe.c
+++ b/ReadMe.c
@@ -203,6 +203,7 @@ struct option long_options[] = {
     {"invalid-backup",0,0,InvalidBackup},
     {"array-size", 1, 0, 'Z'},
     {"continue", 0, 0, Continue},
+    {"force",	  0, 0, Force},
 
     /* For Incremental */
     {"rebuild-map", 0, 0, RebuildMapOpt},
@@ -563,6 +564,7 @@ char Help_grow[] =
 "                      : This is useful if all devices have been replaced\n"
 "                      : with larger devices.   Value is in Kilobytes, or\n"
 "                      : the special word 'max' meaning 'as large as possible'.\n"
+"  --force       -f    : Override normal checks and be more forceful\n"
 "  --assume-clean      : When increasing the --size, this flag will avoid\n"
 "                      : a resync of the new space\n"
 "  --chunk=       -c   : Change the chunksize of the array\n"
diff --git a/mdadm.c b/mdadm.c
index c3a265b..821658a 100644
--- a/mdadm.c
+++ b/mdadm.c
@@ -1617,7 +1617,7 @@ int main(int argc, char *argv[])
 		else if (s.size > 0 || s.raiddisks || s.layout_str ||
 			 s.chunk != 0 || s.level != UnSet ||
 			 data_offset != INVALID_SECTORS) {
-			rv = Grow_reshape(devlist->devname, mdfd,
+		    rv = Grow_reshape(devlist->devname, mdfd, c.force,
 					  devlist->next,
 					  data_offset, &c, &s);
 		} else if (array_size == 0)
diff --git a/mdadm.h b/mdadm.h
index 71b8afb..9e00f05 100644
--- a/mdadm.h
+++ b/mdadm.h
@@ -1300,7 +1300,7 @@ extern int autodetect(void);
 extern int Grow_Add_device(char *devname, int fd, char *newdev);
 extern int Grow_addbitmap(char *devname, int fd,
 			  struct context *c, struct shape *s);
-extern int Grow_reshape(char *devname, int fd,
+extern int Grow_reshape(char *devname, int fd, int force,
 			struct mddev_dev *devlist,
 			unsigned long long data_offset,
 			struct context *c, struct shape *s);