btrfs: Allow forced conversion of metadata to dup profile on, multiple devices

Currently, btrfs balance start fails when trying to convert metadata or system chunks to dup profile on filesystems with multiple devices. This requires that a conversion from a multi-device filesystem to a single device filesystem use the following methodology:
1. btrfs balance start -dconvert=single -mconvert=single \
-sconvert=single -f /
2. btrfs device delete / /dev/sdx
3. btrfs balance start -mconvert=dup -sconvert=dup /
This results in a period of time (possibly very long if the devices are big) where you don't have the protection guarantees of multiple copies of metadata chunks.

After applying this patch, one can instead use the following methodology for conversion from a multi-device filesystem to a single device filesystem:
1. btrfs balance start -dconvert=single -mconvert=dup \
-sconvert=dup -f /
2. btrfs device delete / /dev/sdx
This greatly reduces the chances of the operation causing data loss due to a read error during the device delete.

---
 fs/btrfs/volumes.c | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

index 07629e9..38a9522 100644
@@ -3152,10 +3152,8 @@ int btrfs_balance(struct btrfs_balance_control *bctl,
 		num_devices--;
 	}
 	btrfs_dev_replace_unlock(&fs_info->dev_replace);
-	allowed = BTRFS_AVAIL_ALLOC_BIT_SINGLE;
-	if (num_devices == 1)
-		allowed |= BTRFS_BLOCK_GROUP_DUP;
-	else if (num_devices > 1)
+	allowed = BTRFS_AVAIL_ALLOC_BIT_SINGLE | BTRFS_BLOCK_GROUP_DUP;
+	if (num_devices > 1)
 		allowed |= (BTRFS_BLOCK_GROUP_RAID0 | BTRFS_BLOCK_GROUP_RAID1);
 	if (num_devices > 2)
 		allowed |= BTRFS_BLOCK_GROUP_RAID5;

@@ -3221,6 +3219,21 @@ int btrfs_balance(struct btrfs_balance_control *bctl,
 				goto out;
 			}
 		}
+		if (((bctl->sys.flags & BTRFS_BALANCE_ARGS_CONVERT) &&
+		    (bctl->sys.target & ~BTRFS_BLOCK_GROUP_DUP) ||
+		    (bctl->meta.flags & BTRFS_BALANCE_ARGS_CONVERT) &&
+		    (bctl->meta.target & ~BTRFS_BLOCK_GROUP_DUP)) &&
+		    (num_devs > 1)) {
+			if (bctl->flags & BTRFS_BALANCE_FORCE) {
+				btrfs_info(fs_info, "force conversion of metadata "
+					   "to dup profile on multiple devices");
+			} else {
+				btrfs_err(fs_info, "balance will reduce metadata "
+					  "integrity, use force if you want this");
+				ret = -EINVAL;
+				goto out;
+			}
+		}
 	} while (read_seqretry(&fs_info->profiles_lock, seq));
 
 	if (bctl->sys.flags & BTRFS_BALANCE_ARGS_CONVERT) {

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Fails to build, wait for me to push an updated btrfs-next and rebase and resubmit

fs/btrfs/volumes.c: In function ‘btrfs_balance’:
fs/btrfs/volumes.c:3223:55: warning: suggest parentheses around ‘&&’ within ‘||’ [-Wparentheses]
if (((bctl->sys.flags & BTRFS_BALANCE_ARGS_CONVERT) &&

                                                       ^
fs/btrfs/volumes.c:3227:8: error: ‘num_devs’ undeclared (first use in
this function)
       (num_devs > 1)) {
        ^

fs/btrfs/volumes.c:3227:8: note: each undeclared identifier is reported only once for each function it appears in make[1]: *** [fs/btrfs/volumes.o] Error 1
make: *** [_module_fs/btrfs] Error 2

Thanks,

Josef

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