btrfs: zoned: limit number of zones reclaimed in flush_space()

btrfs: zoned: limit number of zones reclaimed in flush_space()

Limit the number of zones reclaimed in flush_space()'s RECLAIM_ZONES
state.

This prevents possibly long running reclaim sweeps to block other tasks in
the system, while the system is under pressure anyways, causing the
tasks to hang.

An example of this can be seen here, triggered by fstests generic/551:

generic/551        [   27.042349] run fstests generic/551 at 2026-02-27 11:05:30
 BTRFS: device fsid 78c16e29-20d9-4c8e-bc04-7ba431be38ff devid 1 transid 8 /dev/vdb (254:16) scanned by mount (806)
 BTRFS info (device vdb): first mount of filesystem 78c16e29-20d9-4c8e-bc04-7ba431be38ff
 BTRFS info (device vdb): using crc32c checksum algorithm
 BTRFS info (device vdb): host-managed zoned block device /dev/vdb, 64 zones of 268435456 bytes
 BTRFS info (device vdb): zoned mode enabled with zone size 268435456
 BTRFS info (device vdb): checking UUID tree
 BTRFS info (device vdb): enabling free space tree
 INFO: task kworker/u38:1:90 blocked for more than 120 seconds.
       Not tainted 7.0.0-rc1+ #345
 "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
 task:kworker/u38:1   state:D stack:0     pid:90    tgid:90    ppid:2      task_flags:0x4208060 flags:0x00080000
 Workqueue: events_unbound btrfs_async_reclaim_data_space
 Call Trace:
  <TASK>
  __schedule+0x34f/0xe70
  schedule+0x41/0x140
  schedule_timeout+0xa3/0x110
  ? mark_held_locks+0x40/0x70
  ? lockdep_hardirqs_on_prepare+0xd8/0x1c0
  ? trace_hardirqs_on+0x18/0x100
  ? lockdep_hardirqs_on+0x84/0x130
  ? _raw_spin_unlock_irq+0x33/0x50
  wait_for_completion+0xa4/0x150
  ? __flush_work+0x24c/0x550
  __flush_work+0x339/0x550
  ? __pfx_wq_barrier_func+0x10/0x10
  ? wait_for_completion+0x39/0x150
  flush_space+0x243/0x660
  ? find_held_lock+0x2b/0x80
  ? kvm_sched_clock_read+0x11/0x20
  ? local_clock_noinstr+0x17/0x110
  ? local_clock+0x15/0x30
  ? lock_release+0x1b7/0x4b0
  do_async_reclaim_data_space+0xe8/0x160
  btrfs_async_reclaim_data_space+0x19/0x30
  process_one_work+0x20a/0x5f0
  ? lock_is_held_type+0xcd/0x130
  worker_thread+0x1e2/0x3c0
  ? __pfx_worker_thread+0x10/0x10
  kthread+0x103/0x150
  ? __pfx_kthread+0x10/0x10
  ret_from_fork+0x20d/0x320
  ? __pfx_kthread+0x10/0x10
  ret_from_fork_asm+0x1a/0x30
  </TASK>

 Showing all locks held in the system:
 1 lock held by khungtaskd/67:
  #0: ffffffff824d58e0 (rcu_read_lock){....}-{1:3}, at: debug_show_all_locks+0x3d/0x194
 2 locks held by kworker/u38:1/90:
  #0: ffff8881000aa158 ((wq_completion)events_unbound){+.+.}-{0:0}, at: process_one_work+0x3c4/0x5f0
  #1: ffffc90000c17e58 ((work_completion)(&fs_info->async_data_reclaim_work)){+.+.}-{0:0}, at: process_one_work+0x1c0/0x5f0
 5 locks held by kworker/u39:1/191:
  #0: ffff8881000aa158 ((wq_completion)events_unbound){+.+.}-{0:0}, at: process_one_work+0x3c4/0x5f0
  #1: ffffc90000dfbe58 ((work_completion)(&fs_info->reclaim_bgs_work)){+.+.}-{0:0}, at: process_one_work+0x1c0/0x5f0
  #2: ffff888101da0420 (sb_writers#9){.+.+}-{0:0}, at: process_one_work+0x20a/0x5f0
  #3: ffff88811040a648 (&fs_info->reclaim_bgs_lock){+.+.}-{4:4}, at: btrfs_reclaim_bgs_work+0x1de/0x770
  #4: ffff888110408a18 (&fs_info->cleaner_mutex){+.+.}-{4:4}, at: btrfs_relocate_block_group+0x95a/0x20f0
 1 lock held by aio-dio-write-v/980:
  #0: ffff888110093008 (&sb->s_type->i_mutex_key#15){++++}-{4:4}, at: btrfs_inode_lock+0x51/0xb0

 =============================================

To prevent these long running reclaims from blocking the system, only
reclaim 5 block_groups in the RECLAIM_ZONES state of flush_space(). Also
as these reclaims are now constrained, it opens up the use for a
synchronous call to brtfs_reclaim_block_groups(), eliminating the need
to place the reclaim task on a workqueue and then flushing the workqueue
again.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>