This mainline commit, recently backported to [B,F,G] for this bug:
    "btrfs: extent-tree: kill BUG_ON() in __btrfs_free_extent()"
introduces a compile warning.

The warning has been fixed in mainline by:
    cad69d139651 btrfs: tree-checker: fix incorrect printk format
so let's pick up that fix too.

Patch submitted: https://lists.ubuntu.com/archives/kernel-
team/2020-November/114752.html

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https://bugs.launchpad.net/bugs/1902254

Title:
  Bionic: btrfs: kernel BUG at /build/linux-
  eTBZpZ/linux-4.15.0/fs/btrfs/ctree.c:3233!

Status in linux package in Ubuntu:
  Fix Committed
Status in linux source package in Bionic:
  Fix Committed
Status in linux source package in Focal:
  Fix Committed
Status in linux source package in Groovy:
  Fix Committed

Bug description:
  [Impact]

   * Users of btrfs started hitting a kernel BUG() (below)
     after upgrade from 4.15.0-99.100 to 4.15.0-109.110,
     which has 55 btrfs changes.

       kernel BUG at /build/linux-eTBZpZ/linux-4.15.0/fs/btrfs/ctree.c:3233!
       ...
       Krnl PSW : 00000000be9cb874 00000000ef3786e8 
(btrfs_set_item_key_safe+0x152/0x1c0 [btrfs])
       ...
       [...] Call Trace:
       [...] btrfs_set_item_key_safe+0x11c/0x1c0 [btrfs])
       [...] __btrfs_drop_extents+0xb5a/0xda8 [btrfs]
       [...] btrfs_log_changed_extents+0x35c/0xaf0 [btrfs]
       [...] btrfs_log_inode+0x9ee/0x1080 [btrfs]
       [...] btrfs_log_inode_parent+0x224/0xa10 [btrfs]
       [...] btrfs_log_dentry_safe+0x80/0xa8 [btrfs]
       [...] btrfs_sync_file+0x392/0x550 [btrfs]
       [...] do_fsync+0x5e/0x90
       [...] SyS_fdatasync+0x32/0x48
       [...] system_call+0xd8/0x2c8

       $ git log --oneline Ubuntu-4.15.0-99.100..Ubuntu-4.15.0-109.110 -- 
fs/btrfs/ | wc -l
       55

   * The error happens at random moments, regardless of a
     particular activity/load. Workaround is to downgrade.

  [Fix]

   * This BUG()/function is addressed in patch 4/4 [1] of series
     'btrfs: Enhanced runtime defence against fuzzed images' [2],
     after issues in the real world, not just crafted fs images:
     'one internal report has hit one BUG_ON() with real world fs'
   
       kernel BUG at fs/btrfs/ctree.c:3188!
       ...
       RIP: 0010:btrfs_set_item_key_safe+0x16c/0x180
   
   * The patch/set [3] is applied in v5.10-rc1 and Ubuntu Unstable:
     - d16c702fe4f2 btrfs: ctree: check key order before merging tree blocks
     - 07cce5cf3b48 btrfs: extent-tree: kill the BUG_ON() in 
insert_inline_extent_backref()
     - 1c2a07f598d5 btrfs: extent-tree: kill BUG_ON() in __btrfs_free_extent()
     - f98b6215d7d1 btrfs: extent_io: do extra check for extent buffer read 
write functions
   
  [Test Case]

   * There is working synthetic reproducer for this issue,
     which is hard to reproduce as reported in commit [4]
     that introduces debugging for the issue.
     
   * Regression tests with xfstests and stress-ng shows
     no regressions between un/patched kernels.

  [Other Info]

   * Trivial backports (only refreshing a few context lines)
     with 3 more dependency patches on Bionic and 1 on Focal.
     And Bionic needed one extra hunk to '#include' a header.
     Groovy all apply cleanly.

  [1] https://lore.kernel.org/linux-btrfs/20200819063550.62832-5-...@suse.com/
  [2] https://lore.kernel.org/linux-btrfs/20200819063550.62832-1-...@suse.com/
  [3] 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d16c702fe4f274bd77b47d3ab737eadcf24e0b93
  [4] 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7c15d41016dc886cc011e3854d855e219759ae68

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