When doing a buffered write we always try to reserve data space for it,
even when the file has the NOCOW bit set or the write falls into a file
range covered by a prealloc extent. This is done both because it is
expensive to check if we can do a nocow write (checking if an extent is
shared through reflinks or if there's a hole in the range for example),
and because when writeback starts we might actually need to fallback to
COW mode (for example the block group containing the target extents was
turned into RO mode due to a scrub or balance).

When we are unable to reserve data space we check if we can do a nocow
write, and if we can, we proceed with dirtying the pages and setting up
the range for delalloc. In this case the bytes_may_use counter of the
data space_info object is not incremented, unlike in the case where we
are able to reserve data space (done through btrfs_check_data_free_space()
which calls btrfs_alloc_data_chunk_ondemand()).

Later when running delalloc we attempt to start writeback in nocow mode
but we might revert back to cow mode, for example because in the meanwhile
a block group was turned into RO mode by a scrub or relocation. The cow
path after successfully allocating an extent ends up calling
btrfs_add_reserved_bytes(), which expects the bytes_may_use counter of
the data space_info object to have been incremented before - but we did
not do it when the buffered write started, since there was not enough
available data space. So btrfs_add_reserved_bytes() ends up decrementing
the bytes_may_use counter anyway, and when the counter's current value
is smaller then the size of the allocated extent we get a stack trace
like the following:

 ------------[ cut here ]------------
 WARNING: CPU: 0 PID: 20138 at fs/btrfs/space-info.h:115 btrfs_add_reserved_bytes+0x3d6/0x4e0 [btrfs]
 Modules linked in: btrfs blake2b_generic xor raid6_pq libcrc32c (...)
 CPU: 0 PID: 20138 Comm: kworker/u8:15 Not tainted 5.6.0-rc7-btrfs-next-58 #5
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.0-59-gc9ba5276e321-prebuilt.qemu.org 04/01/2014
 Workqueue: writeback wb_workfn (flush-btrfs-1754)
 RIP: 0010:btrfs_add_reserved_bytes+0x3d6/0x4e0 [btrfs]
 Code: ff ff 48 (...)
 RSP: 0018:ffffbda18a4b3568 EFLAGS: 00010287
 RAX: 0000000000000000 RBX: ffff9ca076f5d800 RCX: 0000000000000000
 RDX: 0000000000000002 RSI: 0000000000000000 RDI: ffff9ca068470410
 RBP: fffffffffffff000 R08: 0000000000000001 R09: 0000000000000000
 R10: ffff9ca079d58040 R11: 0000000000000000 R12: ffff9ca068470400
 R13: ffff9ca0408b2000 R14: 0000000000001000 R15: ffff9ca076f5d800
 FS:  0000000000000000(0000) GS:ffff9ca07a600000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 00005605dbfe7048 CR3: 0000000138570006 CR4: 00000000003606f0
 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
 Call Trace:
  find_free_extent+0x4a0/0x16c0 [btrfs]
  btrfs_reserve_extent+0x91/0x180 [btrfs]
  cow_file_range+0x12d/0x490 [btrfs]
  run_delalloc_nocow+0x341/0xa40 [btrfs]
  btrfs_run_delalloc_range+0x1ea/0x6d0 [btrfs]
  ? find_lock_delalloc_range+0x221/0x250 [btrfs]
  writepage_delalloc+0xe8/0x150 [btrfs]
  __extent_writepage+0xe8/0x4c0 [btrfs]
  extent_write_cache_pages+0x237/0x530 [btrfs]
  ? btrfs_wq_submit_bio+0x9f/0xc0 [btrfs]
  extent_writepages+0x44/0xa0 [btrfs]
  do_writepages+0x23/0x80
  __writeback_single_inode+0x59/0x700
  writeback_sb_inodes+0x267/0x5f0
  __writeback_inodes_wb+0x87/0xe0
  wb_writeback+0x382/0x590
  ? wb_workfn+0x4a2/0x6c0
  wb_workfn+0x4a2/0x6c0
  process_one_work+0x26d/0x6a0
  worker_thread+0x4f/0x3e0
  ? process_one_work+0x6a0/0x6a0
  kthread+0x103/0x140
  ? kthread_create_worker_on_cpu+0x70/0x70
  ret_from_fork+0x3a/0x50
 irq event stamp: 0
 hardirqs last  enabled at (0): [<0000000000000000>] 0x0
 hardirqs last disabled at (0): [<ffffffff94ebdedf>] copy_process+0x74f/0x2020
 softirqs last  enabled at (0): [<ffffffff94ebdedf>] copy_process+0x74f/0x2020
 softirqs last disabled at (0): [<0000000000000000>] 0x0
 ---[ end trace f9f6ef8ec4cd8ec9 ]---

So to fix this, when falling back into cow mode check if space was not
reserved, by testing for the bit EXTENT_NORESERVE in the respective file
range, and if not, increment the bytes_may_use counter for the data
space_info object. Also clear the EXTENT_NORESERVE bit from the range, so
that if the cow path fails it decrements the bytes_may_use counter when
clearing the delalloc range (through the btrfs_clear_delalloc_extent()
callback).

Fixes: 7ee9e440 ("Btrfs: check if we can nocow if we don't have data space")
CC: stable@vger.kernel.org # 4.4+
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>