Not familiar about the problem, but you don't really need to install the 
patch.

You can just execute '<your btrfs-progs source dir>/btrfsck' directly to 
call the new fsck.

And there is still some time before the patch is merged, there is really 
no need to install.

Thanks,
Qu

Ivan P wrote on 2016/05/17 20:12 +0200:

Thank you, however I can't seem to be able to compile that snapshot, I'm getting

===========================
/usr/bin/install -c -m644 -d 64-btrfs-dm.rules
/home/myuser/aur/btrfs-progs-git/pkg/btrfs-progs-git/usr/lib/udev/rules.d
/usr/bin/install: cannot create directory ‘64-btrfs-dm.rules’: File exists
Makefile:400: recipe for target 'install' failed
make: *** [install] Error 1
==> ERROR: A failure occurred in package()
===========================

Just to make sure I wasn't screwing up somewhere, I tried the
btrfs-progs-git AUR package and I'm getting the same thing.
It's not only me, however: according to [1] it could be that this
commit has introduced it: [2]

Regards,
Ivan.

[1] https://aur.archlinux.org/packages/btrfs-progs-git/
[2] http://repo.or.cz/btrfs-progs-unstable/devel.git?a=commit;h=ebe5b1cc7885027521db3c1d16d84bd54cc1321b

On Tue, May 17, 2016 at 5:56 AM, Qu Wenruo [off-list ref] wrote:

quoted

Also to Ivan P, the initial reporter of the problem.

Now "btrfsck --clear-cache" should help you to prevent kernel warning on
free space cache problem.

Thanks,
Qu

Qu Wenruo wrote on 2016/05/17 11:47 +0800:

quoted

The patchset can be also fetched from github, just in case mail list
blocks the last patch, which contains binary file.
https://github.com/adam900710/btrfs-progs.git fsck_clear_cache

Just as describe in the first patch, btrfs kernel "clear_cache" mount
option can't rebuild all free space cache, due to the design of free
space cache.
(Although I pretty like the idea to delay the load of free space cache,
as it hugely reduce the mount time of large fs)

So this makes users to report error in mail list, complaining
"clear_cache" doesn't wipe the annoying kernel warning on corrupted free
space cache.

Since kernel can't handle it, and user consider it more like an error,
we'd better handle it like an error, to fix it in btrfs check.

So this patchset adds the ability to clear free space cache, and add
test case for it.

The clear procedure is different from kernel, it will remove all free
space cache inodes and its contents(with backrefs), and set
cache_generation of super block to -1.
So there will be no free space cache at all and kernel will be happy
with that.

This patch also enhances btrfs_previous_item() to use min_objectid to
return as early as possible.

Lu Fengqi (1):
  btrfs-progs: tests: add 020-bad-free-space-cache

Qu Wenruo (3):
  btrfs-progs: corrupt-block: Add ability to corrupt free space cache
    file
  btrfs-progs: ctree: return earlier for btrfs_previous_item
  btrfs-progs: fsck: Add support to clear free space cache

 Documentation/btrfs-check.asciidoc                 |   8 ++
 btrfs-corrupt-block.c                              | 124
++++++++++++++++++++-
 cmds-check.c                                       |  58 +++++++++-
 ctree.c                                            |   2 +
 free-space-cache.c                                 | 124
+++++++++++++++++++++
 free-space-cache.h                                 |   4 +
 .../020-bad-free-space-cache/default_case.raw.xz   | Bin 0 -> 164068
bytes
 tests/fsck-tests/020-bad-free-space-cache/test.sh  |  16 +++
 8 files changed, 334 insertions(+), 2 deletions(-)
 create mode 100644
tests/fsck-tests/020-bad-free-space-cache/default_case.raw.xz
 create mode 100755 tests/fsck-tests/020-bad-free-space-cache/test.sh