Btrfs: fix access to available allocation bits when starting balance

From: Filipe Manana <[email protected]>

The available allocation bits members from struct btrfs_fs_info are protected by a sequence lock, and when starting balance we access them incorrectly in two different ways:

1) In the read sequence lock loop at btrfs_balance() we use the values we read from fs_info->avail_*_alloc_bits and we can immediately do actions that have side effects and can not be undone (printing a message and jumping to a label). This is wrong because a retry might be needed, so our actions must not have side effects and must be repeatable as long as read_seqretry() returns a non-zero value. In other words, we were essentially ignoring the sequence lock;

2) Right below the read sequence lock loop, we were reading the values from avail_metadata_alloc_bits and avail_data_alloc_bits without any protection from concurrent writers, that is, reading them outside of the read sequence lock critical section.

So fix this by making sure we only read the available allocation bits while in a read sequence lock critical section and that what we do in the critical section is repeatable (has nothing that can not be undone) so that any eventual retry that is needed is handled properly.

Fixes de98ced9e743 ("Btrfs: use seqlock to protect fs_info->avail_{data, metadata, system}_alloc_bits")

Fixes 14506127979a ("btrfs: fix a bogus warning when converting only data or metadata")

Signed-off-by Filipe Manana <[email protected]>

---
 fs/btrfs/volumes.c | 39 +++++++++++++++++++++++----------------
 1 file changed, 23 insertions(+), 16 deletions(-)

fs/btrfs/volumes.c +23 -16

index f4405e430da6..223334f08530 100644
@@ -3712,6 +3712,7 @@ int btrfs_balance(struct btrfs_fs_info *fs_info,
 	int ret;
 	u64 num_devices;
 	unsigned seq;
+	bool reducing_integrity;
 
 	if (btrfs_fs_closing(fs_info) ||
 	    atomic_read(&fs_info->balance_pause_req) ||

@@ -3796,24 +3797,30 @@ int btrfs_balance(struct btrfs_fs_info *fs_info,
 		     !(bctl->sys.target & allowed)) ||
 		    ((bctl->meta.flags & BTRFS_BALANCE_ARGS_CONVERT) &&
 		     (fs_info->avail_metadata_alloc_bits & allowed) &&
-		     !(bctl->meta.target & allowed))) {
-			if (bctl->flags & BTRFS_BALANCE_FORCE) {
-				btrfs_info(fs_info,
-				"balance: force reducing metadata integrity");
-			} else {
-				btrfs_err(fs_info,
-	"balance: reduces metadata integrity, use --force if you want this");
-				ret = -EINVAL;
-				goto out;
-			}
-		}
+		     !(bctl->meta.target & allowed)))
+			reducing_integrity = true;
+		else
+			reducing_integrity = false;
+
+		/* if we're not converting, the target field is uninitialized */
+		meta_target = (bctl->meta.flags & BTRFS_BALANCE_ARGS_CONVERT) ?
+			bctl->meta.target : fs_info->avail_metadata_alloc_bits;
+		data_target = (bctl->data.flags & BTRFS_BALANCE_ARGS_CONVERT) ?
+			bctl->data.target : fs_info->avail_data_alloc_bits;
 	} while (read_seqretry(&fs_info->profiles_lock, seq));
 
-	/* if we're not converting, the target field is uninitialized */
-	meta_target = (bctl->meta.flags & BTRFS_BALANCE_ARGS_CONVERT) ?
-		bctl->meta.target : fs_info->avail_metadata_alloc_bits;
-	data_target = (bctl->data.flags & BTRFS_BALANCE_ARGS_CONVERT) ?
-		bctl->data.target : fs_info->avail_data_alloc_bits;
+	if (reducing_integrity) {
+		if (bctl->flags & BTRFS_BALANCE_FORCE) {
+			btrfs_info(fs_info,
+				   "balance: force reducing metadata integrity");
+		} else {
+			btrfs_err(fs_info,
+	  "balance: reduces metadata integrity, use --force if you want this");
+			ret = -EINVAL;
+			goto out;
+		}
+	}
+
 	if (btrfs_get_num_tolerated_disk_barrier_failures(meta_target) <
 		btrfs_get_num_tolerated_disk_barrier_failures(data_target)) {
 		int meta_index = btrfs_bg_flags_to_raid_index(meta_target);

From: Filipe Manana <[email protected]>

The available allocation bits members from struct btrfs_fs_info are protected by a sequence lock, and when starting balance we access them incorrectly in two different ways:

1) In the read sequence lock loop at btrfs_balance() we use the values we read from fs_info->avail_*_alloc_bits and we can immediately do actions that have side effects and can not be undone (printing a message and jumping to a label). This is wrong because a retry might be needed, so our actions must not have side effects and must be repeatable as long as read_seqretry() returns a non-zero value. In other words, we were essentially ignoring the sequence lock;

2) Right below the read sequence lock loop, we were reading the values from avail_metadata_alloc_bits and avail_data_alloc_bits without any protection from concurrent writers, that is, reading them outside of the read sequence lock critical section.

So fix this by making sure we only read the available allocation bits while in a read sequence lock critical section and that what we do in the critical section is repeatable (has nothing that can not be undone) so that any eventual retry that is needed is handled properly.

Fixes de98ced9e743 ("Btrfs: use seqlock to protect fs_info->avail_{data, metadata, system}_alloc_bits")

Fixes 14506127979a ("btrfs: fix a bogus warning when converting only data or metadata")

Signed-off-by Filipe Manana <[email protected]>