btrfs: free anon block device right after subvolume deletion

commit 082b6c970f02fefd278c7833880cda29691a5f34 upstream.

[BUG]
When a lot of subvolumes are created, there is a user report about
transaction aborted caused by slow anonymous block device reclaim:

  BTRFS: Transaction aborted (error -24)
  WARNING: CPU: 17 PID: 17041 at fs/btrfs/transaction.c:1576 create_pending_snapshot+0xbc4/0xd10 [btrfs]
  RIP: 0010:create_pending_snapshot+0xbc4/0xd10 [btrfs]
  Call Trace:
   create_pending_snapshots+0x82/0xa0 [btrfs]
   btrfs_commit_transaction+0x275/0x8c0 [btrfs]
   btrfs_mksubvol+0x4b9/0x500 [btrfs]
   btrfs_ioctl_snap_create_transid+0x174/0x180 [btrfs]
   btrfs_ioctl_snap_create_v2+0x11c/0x180 [btrfs]
   btrfs_ioctl+0x11a4/0x2da0 [btrfs]
   do_vfs_ioctl+0xa9/0x640
   ksys_ioctl+0x67/0x90
   __x64_sys_ioctl+0x1a/0x20
   do_syscall_64+0x5a/0x110
   entry_SYSCALL_64_after_hwframe+0x44/0xa9
  ---[ end trace 33f2f83f3d5250e9 ]---
  BTRFS: error (device sda1) in create_pending_snapshot:1576: errno=-24 unknown
  BTRFS info (device sda1): forced readonly
  BTRFS warning (device sda1): Skipping commit of aborted transaction.
  BTRFS: error (device sda1) in cleanup_transaction:1831: errno=-24 unknown

[CAUSE]
The anonymous device pool is shared and its size is 1M. It's possible to
hit that limit if the subvolume deletion is not fast enough and the
subvolumes to be cleaned keep the ids allocated.

[WORKAROUND]
We can't avoid the anon device pool exhaustion but we can shorten the
time the id is attached to the subvolume root once the subvolume becomes
invisible to the user.

Reported-by: Greed Rong <greedrong@gmail.com>
Link: https://lore.kernel.org/linux-btrfs/CA+UqX+NTrZ6boGnWHhSeZmEY5J76CTqmYjO2S+=tHJX7nb9DPw@mail.gmail.com/
CC: stable@vger.kernel.org # 4.4+
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Qu Wenruo <wqu@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>
(cherry picked from commit d2731ac13ca5a7addd4cd4fa65d3fae5463b795f)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 57b324dd528d4b0abcde136b2fb25c0f3fe47090..7d0bcb83b64e9818a3035bead915f4086e39c805 100644 (file)
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -4032,6 +4032,8 @@ int btrfs_delete_subvolume(struct inode *dir, struct dentry *dentry)
                }
        }
 
+       free_anon_bdev(dest->anon_dev);
+       dest->anon_dev = 0;
 out_end_trans:
        trans->block_rsv = NULL;
        trans->bytes_reserved = 0;