dm cache: make smq background work limit configurable

The maximum number of concurrent background work items (promotions,
demotions, writebacks) in the SMQ policy was hardcoded to 4096, with
a FIXME comment noting it should be made configurable.

This value was originally tuned down from 10240 to balance memory
overhead (~128 bytes per entry, ~512KB at 4096 entries) against I/O
parallelism. However, different workloads and cache sizes may benefit
from different limits:

- Write-heavy workloads may need more writeback concurrency
- Very large caches (10+ TB) may need more promotion slots
- Memory-constrained systems may want a lower limit

Make this configurable via the module parameter "smq_max_background_work"
(defaulting to 4096 to preserve existing behaviour). Clamp the value to
at least 1 to prevent setting 0, which would block all background work.
The parameter only affects newly created cache devices; existing caches
retain their value from creation time.

Signed-off-by: Cao Guanghui <caoguanghui@kylinos.cn>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

Author: Cao Guanghui

drivers/md/dm-cache-policy-smq.c

@@ -21,6 +21,16 @@
 
 /*----------------------------------------------------------------*/
 
+/*
+ * Maximum number of concurrent background work items (promotions,
+ * demotions, writebacks) that can be queued in the background tracker.
+ * Tuneable via the module parameter smq_max_background_work.
+ * Only affects newly created cache devices.
+ */
+static unsigned int smq_max_background_work = 4096;
+module_param(smq_max_background_work, uint, 0644);
+MODULE_PARM_DESC(smq_max_background_work, "Max concurrent background work items");
+
 /*
  * Safe division functions that return zero on divide by zero.
  */
@@ -1816,7 +1826,7 @@ __smq_create(dm_cblock_t cache_size, sector_t origin_size, sector_t cache_block_
 	mq->next_hotspot_period = jiffies;
 	mq->next_cache_period = jiffies;
 
-	mq->bg_work = btracker_create(4096); /* FIXME: hard coded value */
+	mq->bg_work = btracker_create(max(1u, smq_max_background_work));
 	if (!mq->bg_work)
 		goto bad_btracker;