ubi: fastmap: Fix high cpu usage of ubi_bgt by making sure wl_pool not empty

There at least 6 PEBs reserved on UBI device:
1. EBA_RESERVED_PEBS[1]
2. WL_RESERVED_PEBS[1]
3. UBI_LAYOUT_VOLUME_EBS[2]
4. MIN_FASTMAP_RESERVED_PEBS[2]

When all ubi volumes take all their PEBs, there are 3 (EBA_RESERVED_PEBS +
WL_RESERVED_PEBS + MIN_FASTMAP_RESERVED_PEBS - MIN_FASTMAP_TAKEN_PEBS[1])
free PEBs. Since commit f9c34bb5 ("ubi: Fix producing anchor PEBs")
and commit 4b68bf9a ("ubi: Select fastmap anchor PEBs considering
wear level rules") applied, there is only 1 (3 - FASTMAP_ANCHOR_PEBS[1] -
FASTMAP_NEXT_ANCHOR_PEBS[1]) free PEB to fill pool and wl_pool, after
filling pool, wl_pool is always empty. So, UBI could be stuck in an
infinite loop:

	ubi_thread	   system_wq
wear_leveling_worker <--------------------------------------------------
  get_peb_for_wl							|
    // fm_wl_pool, used = size = 0					|
    schedule_work(&ubi->fm_work)					|
									|
		    update_fastmap_work_fn				|
		      ubi_update_fastmap				|
			ubi_refill_pools				|
			// ubi->free_count - ubi->beb_rsvd_pebs < 5	|
			// wl_pool is not filled with any PEBs		|
			schedule_erase(old_fm_anchor)			|
			ubi_ensure_anchor_pebs				|
			  __schedule_ubi_work(wear_leveling_worker)	|
									|
__erase_worker								|
  ensure_wear_leveling							|
    __schedule_ubi_work(wear_leveling_worker) --------------------------

, which cause high cpu usage of ubi_bgt:
top - 12:10:42 up 5 min,  2 users,  load average: 1.76, 0.68, 0.27
Tasks: 123 total,   3 running,  54 sleeping,   0 stopped,   0 zombie

  PID USER PR   NI VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND
 1589 root 20   0   0      0      0 R  45.0  0.0   0:38.86 ubi_bgt0d
  319 root 20   0   0      0      0 I  15.2  0.0   0:15.29 kworker/0:3-eve
  371 root 20   0   0      0      0 I  14.9  0.0   0:12.85 kworker/3:3-eve
   20 root 20   0   0      0      0 I  11.3  0.0   0:05.33 kworker/1:0-eve
  202 root 20   0   0      0      0 I  11.3  0.0   0:04.93 kworker/2:3-eve

In commit 4b68bf9a ("ubi: Select fastmap anchor PEBs considering
wear level rules"), there are three key changes:
  1) Choose the fastmap anchor when the most free PEBs are available.
  2) Enable anchor move within the anchor area again as it is useful
     for distributing wear.
  3) Import a candidate fm anchor and check this PEB's erase count during
     wear leveling. If the wear leveling limit is exceeded, use the used
     anchor area PEB with the lowest erase count to replace it.

The anchor candidate can be removed, we can check fm_anchor PEB's erase
count during wear leveling. Fix it by:
  1) Removing 'fm_next_anchor' and check 'fm_anchor' during wear leveling.
  2) Preferentially filling one free peb into fm_wl_pool in condition of
     ubi->free_count > ubi->beb_rsvd_pebs, then try to reserve enough
     free count for fastmap non anchor pebs after the above prerequisites
     are met.
Then, there are at least 1 PEB in pool and 1 PEB in wl_pool after calling
ubi_refill_pools() with all erase works done.

Fetch a reproducer in [Link].

Fixes: 4b68bf9a ("ubi: Select fastmap anchor PEBs ... rules")
Link: https://bugzilla.kernel.org/show_bug.cgi?id=215407


Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>