[PATCH 2/8] md/raid5: raise stripe cache limit from 32768 to 262144

[Hiroshi Nishida <nishidafmly@gmail.com>]

Stripe cache memory is approximately max_nr_stripes * num_disks *
stripe_size, where stripe_size is 4KB (PAGE_SIZE).  The old hardcoded
ceiling of 32768 stripes therefore limits a 12-disk array to ~1.5GB of
stripe cache.  On servers with 256GB+ RAM backing wide arrays, a larger
cache reduces stalls on stripe-cache misses under heavy write workloads.

Define RAID5_MAX_NR_STRIPES = 262144 (256K) in raid5.h and use it in
raid5_set_cache_size() instead of the magic 32768 literal.  This allows
up to ~12GB of stripe cache on a 12-disk array.  The default stripe count
(NR_STRIPES = 256) is unchanged, so there is no memory impact unless the
administrator raises stripe_cache_size explicitly.

Also fix two local variables in raid5_cache_count() that were declared
as int but read from unsigned int fields; change to unsigned int to
avoid implicit sign-extension in the subtraction.

Tested: echo 262144 > /sys/block/md0/md/stripe_cache_size accepted;
262145 correctly returns EINVAL.

Assisted-by: Claude:claude-opus-4-8 [Claude Code]
Signed-off-by: Hiroshi Nishida <nishidafmly@gmail.com>
---
 drivers/md/raid5.c | 6 +++---
 drivers/md/raid5.h | 6 ++++++
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 28828e083c2b..9cb4ed3bd85c 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -6923,7 +6923,7 @@ raid5_set_cache_size(struct mddev *mddev, int size)
 	int result = 0;
 	struct r5conf *conf = mddev->private;
 
-	if (size <= 16 || size > 32768)
+	if (size <= 16 || size > RAID5_MAX_NR_STRIPES)
 		return -EINVAL;
 
 	WRITE_ONCE(conf->min_nr_stripes, size);
@@ -7505,8 +7505,8 @@ static unsigned long raid5_cache_count(struct shrinker *shrink,
 				       struct shrink_control *sc)
 {
 	struct r5conf *conf = shrink->private_data;
-	int max_stripes = READ_ONCE(conf->max_nr_stripes);
-	int min_stripes = READ_ONCE(conf->min_nr_stripes);
+	unsigned int max_stripes = READ_ONCE(conf->max_nr_stripes);
+	unsigned int min_stripes = READ_ONCE(conf->min_nr_stripes);
 
 	if (max_stripes < min_stripes)
 		/* unlikely, but not impossible */
diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
index 5cd9d0f36b6e..57349737d393 100644
--- a/drivers/md/raid5.h
+++ b/drivers/md/raid5.h
@@ -479,6 +479,12 @@ struct disk_info {
  */
 
 #define NR_STRIPES		256
+/* Maximum user-settable stripe cache size, in stripes.  Stripe cache memory is
+ * roughly max_nr_stripes * num_disks * stripe_size (stripe_size = 4KB), so the
+ * old cap of 32768 limited a 12-disk array to ~1.5GB.  Raise to 262144 to allow
+ * larger caches on big-RAM systems; the default (NR_STRIPES) is unchanged.
+ */
+#define RAID5_MAX_NR_STRIPES	262144U
 
 #if PAGE_SIZE == DEFAULT_STRIPE_SIZE
 #define STRIPE_SIZE		PAGE_SIZE
-- 
2.43.0