[PATCH v6 15/15] ubi: fastmap: Fix high cpu usage of ubi_bgt by making sure wl_pool not empty

[Zhihao Cheng]

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 f9c34bb529975fe ("ubi: Fix producing anchor PEBs") and
4b68bf9a69d22dd ("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

The fm_next_anchor always takes one free PEB to prepare for updating
fastmap, fm_anchor and fm_next_anchor are always allocated first of pool
and wl_pool, so UBI can regard these two PEBs as reserved PEBs.

Fix it by:
  1) Adding 2 PEBs reserved for fm_anchor and fm_next_anchor.
  2) Abandoning filling wl_pool until free count belows beb_rsvd_pebs.
Then, there are at least 2(EBA_RESERVED_PEBS + MIN_FASTMAP_RESERVED_PEBS -
MIN_FASTMAP_TAKEN_PEBS[1]) PEBs in pool and 1(WL_RESERVED_PEBS) PEB in
wl_pool after calling ubi_refill_pools() with all erase works done.

This modification will cause a compatibility problem with old UBI image.
If UBI volumes take the maximun number of PEBs for one certain UBI device,
there are no available PEBs to satisfy 2 new reserved PEBs, bad reserved
PEBs are taken firstly, if still not enough, ENOSPC will returned from ubi
initialization.

Fetch a reproducer in [Link].

Fixes: f9c34bb529975fe ("ubi: Fix producing anchor PEBs")
Fixes: 4b68bf9a69d22dd ("ubi: Select fastmap anchor PEBs ... rules")
Link: https://bugzilla.kernel.org/show_bug.cgi?id=215407
Signed-off-by: Zhihao Cheng <chengzhihao1 at huawei.com>
---
 drivers/mtd/ubi/fastmap-wl.c |  5 +++--
 drivers/mtd/ubi/wl.h         | 11 +++++++++--
 2 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/drivers/mtd/ubi/fastmap-wl.c b/drivers/mtd/ubi/fastmap-wl.c
index 28f55f9cf715..18f23e76fee9 100644
--- a/drivers/mtd/ubi/fastmap-wl.c
+++ b/drivers/mtd/ubi/fastmap-wl.c
@@ -134,7 +134,8 @@ void ubi_refill_pools(struct ubi_device *ubi)
 	for (;;) {
 		enough = 0;
 		if (pool->size < pool->max_size) {
-			if (!ubi->free.rb_node)
+			if (!ubi->free.rb_node ||
+			    (ubi->free_count <= ubi->beb_rsvd_pebs))
 				break;
 
 			e = wl_get_wle(ubi);
@@ -148,7 +149,7 @@ void ubi_refill_pools(struct ubi_device *ubi)
 
 		if (wl_pool->size < wl_pool->max_size) {
 			if (!ubi->free.rb_node ||
-			   (ubi->free_count - ubi->beb_rsvd_pebs < 5))
+			    (ubi->free_count <= ubi->beb_rsvd_pebs))
 				break;
 
 			e = find_wl_entry(ubi, &ubi->free, WL_FREE_MAX_DIFF);
diff --git a/drivers/mtd/ubi/wl.h b/drivers/mtd/ubi/wl.h
index c93a53293786..e6c8f68d5ffb 100644
--- a/drivers/mtd/ubi/wl.h
+++ b/drivers/mtd/ubi/wl.h
@@ -2,14 +2,21 @@
 #ifndef UBI_WL_H
 #define UBI_WL_H
 #ifdef CONFIG_MTD_UBI_FASTMAP
+
+/* Number of physical eraseblocks for fm anchor */
+#define FASTMAP_ANCHOR_PEBS 1
+/* Number of physical eraseblocks for next fm anchor */
+#define FASTMAP_NEXT_ANCHOR_PEBS 1
+
 static void update_fastmap_work_fn(struct work_struct *wrk);
 static struct ubi_wl_entry *find_anchor_wl_entry(struct rb_root *root);
 static struct ubi_wl_entry *get_peb_for_wl(struct ubi_device *ubi);
 static void ubi_fastmap_close(struct ubi_device *ubi);
 static inline void ubi_fastmap_init(struct ubi_device *ubi, int *count)
 {
-	/* Reserve enough LEBs to store two fastmaps. */
-	*count += (ubi->fm_size / ubi->leb_size) * 2;
+	/* Reserve enough LEBs to store two fastmaps and anchor fm pebs */
+	*count += (ubi->fm_size / ubi->leb_size) * 2 +
+		  FASTMAP_ANCHOR_PEBS + FASTMAP_NEXT_ANCHOR_PEBS;
 	INIT_WORK(&ubi->fm_work, update_fastmap_work_fn);
 }
 static struct ubi_wl_entry *may_reserve_for_fm(struct ubi_device *ubi,
-- 
2.31.1