[PATCH 2/2] xen/blkback: turn the cache purge percent into a parameter

[Roger Pau Monne <roger.pau@citrix.com>]

Assume that reads and writes to the variable will be atomic. The worse
that could happen is that one of the purges removes a partially
written percentage of grants, but the cache itself will recover.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
---
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: SeongJae Park <sjpark@amazon.de>
Cc: xen-devel@lists.xenproject.org
Cc: linux-block@vger.kernel.org
Cc: J. Roeleveld <joost@antarean.org>
Cc: Jürgen Groß <jgross@suse.com>
---
 Documentation/ABI/testing/sysfs-driver-xen-blkback | 9 +++++++++
 drivers/block/xen-blkback/blkback.c                | 7 +++++--
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-driver-xen-blkback b/Documentation/ABI/testing/sysfs-driver-xen-blkback
index 776f25d335ca..7de791ad61f9 100644
--- a/Documentation/ABI/testing/sysfs-driver-xen-blkback
+++ b/Documentation/ABI/testing/sysfs-driver-xen-blkback
@@ -45,3 +45,12 @@ Description:
                 to be executed periodically. This parameter controls the time
                 interval between consecutive executions of the purge mechanism
                 is set in ms.
+
+What:           /sys/module/xen_blkback/parameters/lru_percent_clean
+Date:           October 2020
+KernelVersion:  5.10
+Contact:        Roger Pau Monné <roger.pau@citrix.com>
+Description:
+                When the persistent grants list is full we will remove unused
+                grants from the list. The percent number of grants to be
+                removed at each LRU execution.
diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index 6ad9b76fdb2b..772852d45a5a 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -127,7 +127,10 @@ MODULE_PARM_DESC(lru_internval,
  * from the list. The percent number of grants to be removed at each LRU
  * execution.
  */
-#define LRU_PERCENT_CLEAN 5
+static unsigned int lru_percent_clean = 5;
+module_param_named(lru_percent_clean, lru_percent_clean, uint, 0644);
+MODULE_PARM_DESC(lru_percent_clean,
+		 "Percentage of persistent grants to remove from the cache when full");
 
 /* Run-time switchable: /sys/module/blkback/parameters/ */
 static unsigned int log_stats;
@@ -404,7 +407,7 @@ static void purge_persistent_gnt(struct xen_blkif_ring *ring)
 	    !ring->blkif->vbd.overflow_max_grants)) {
 		num_clean = 0;
 	} else {
-		num_clean = (max_pgrants / 100) * LRU_PERCENT_CLEAN;
+		num_clean = (max_pgrants / 100) * lru_percent_clean;
 		num_clean = ring->persistent_gnt_c - max_pgrants + num_clean;
 		num_clean = min(ring->persistent_gnt_c, num_clean);
 		pr_debug("Going to purge at least %u persistent grants\n",
-- 
2.28.0