bcachefs: make durability a read-write sysfs option

Sometimes the user may need to change durability after formatting to
match current hardware setup, this option provides a quick and flexible
alternative to removing then adding the device.
It is HIGHLY ADVISED TO RUN REREPLICATE after changing this value so the
system doesn't remain degraded.
Signed-off-by: Daniel Hill <daniel@gluo.nz>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/sysfs.c

@@ -174,7 +174,7 @@ read_attribute(minor);
 read_attribute(bucket_size);
 read_attribute(first_bucket);
 read_attribute(nbuckets);
-read_attribute(durability);
+rw_attribute(durability);
 read_attribute(iodone);
 
 read_attribute(io_latency_read);
@@ -911,6 +911,19 @@ STORE(bch2_dev)
 		mutex_unlock(&c->sb_lock);
 	}
 
+	if (attr == &sysfs_durability) {
+		u64 v = strtoul_or_return(buf);
+
+		mutex_lock(&c->sb_lock);
+		mi = &bch2_sb_get_members(c->disk_sb.sb)->members[ca->dev_idx];
+
+		if (v != BCH_MEMBER_DURABILITY(mi)) {
+			SET_BCH_MEMBER_DURABILITY(mi, v + 1);
+			bch2_write_super(c);
+		}
+		mutex_unlock(&c->sb_lock);
+	}
+
 	if (attr == &sysfs_label) {
 		char *tmp;
 		int ret;