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Subject[PATCH 4.20 55/57] Btrfs: fix access to available allocation bits when starting balance
Date
4.20-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Filipe Manana <fdmanana@suse.com>

commit 5a8067c0d17feb7579db0476191417b441a8996e upstream.

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")
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -3724,6 +3724,7 @@ int btrfs_balance(struct btrfs_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) ||
@@ -3803,24 +3804,30 @@ int btrfs_balance(struct btrfs_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);

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