btrfs: allow unlink to exceed subvolume quota

Once a qgroup limit is exceeded, it's impossible to restore normal
operation to the subvolume without modifying the limit or removing
the subvolume.  This is a surprising situation for many users used
to the typical workflow with quotas on other file systems where it's
possible to remove files until the used space is back under the limit.

When we go to unlink a file and start the transaction, we'll hit
the qgroup limit while trying to reserve space for the items we'll
modify while removing the file.  We discussed last month how best
to handle this situation and agreed that there is no perfect solution.
The best principle-of-least-surprise solution is to handle it similarly
to how we already handle ENOSPC when unlinking, which is to allow
the operation to succeed with the expectation that it will ultimately
release space under most circumstances.

This patch modifies the transaction start path to select whether to
honor the qgroups limits.  btrfs_start_transaction_fallback_global_rsv
is the only caller that skips enforcement.  The reservation and tracking
still happens normally -- it just skips the enforcement step.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Reviewed-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/fs/btrfs/qgroup.h b/fs/btrfs/qgroup.h
index 416ae8e1d23c..ee95f456a61f 100644
--- a/fs/btrfs/qgroup.h
+++ b/fs/btrfs/qgroup.h
@@ -181,7 +181,8 @@ int btrfs_qgroup_reserve_data(struct inode *inode, u64 start, u64 len);
 int btrfs_qgroup_release_data(struct inode *inode, u64 start, u64 len);
 int btrfs_qgroup_free_data(struct inode *inode, u64 start, u64 len);
 
-int btrfs_qgroup_reserve_meta(struct btrfs_root *root, int num_bytes);
+int btrfs_qgroup_reserve_meta(struct btrfs_root *root, int num_bytes,
+			      bool enforce);
 void btrfs_qgroup_free_meta_all(struct btrfs_root *root);
 void btrfs_qgroup_free_meta(struct btrfs_root *root, int num_bytes);
 void btrfs_qgroup_check_reserved_leak(struct inode *inode);