[RFC PATCH v1.1 5/5] mm/damon/sysfs: read ops_id only once

damon_sysfs_apply_inputs() reads ops_id twice.  It could race with
ops_id_store().  As a result, the min_region_sz could wrongly be set up.
Read it once.

The user impact is trivial.  Sane users ain't update the parameter in
parallel.  Even if it happens, only monitoring itself runs differently
than expected.  No critical consequences like kernel panic or memory
corruption happen.

The issue was discovered [1] by Sashiko.

[1] https://lore.kernel.org/[email protected]

Signed-off-by: SJ Park <[email protected]>
---
 mm/damon/sysfs.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
index b5fe036f78015..60a1a9e4ada34 100644
--- a/mm/damon/sysfs.c
+++ b/mm/damon/sysfs.c
@@ -2094,14 +2094,16 @@ static inline bool damon_sysfs_kdamond_running(
 static int damon_sysfs_apply_inputs(struct damon_ctx *ctx,
 		struct damon_sysfs_context *sys_ctx)
 {
+	enum damon_ops_id ops_id;
 	int err;
 
-	err = damon_select_ops(ctx, sys_ctx->ops_id);
+	ops_id = READ_ONCE(sys_ctx->ops_id);
+	err = damon_select_ops(ctx, ops_id);
 	if (err)
 		return err;
 	ctx->addr_unit = sys_ctx->addr_unit;
 	/* addr_unit is respected by only DAMON_OPS_PADDR */
-	if (sys_ctx->ops_id == DAMON_OPS_PADDR)
+	if (ops_id == DAMON_OPS_PADDR)
 		ctx->min_region_sz = max(
 				DAMON_MIN_REGION_SZ / sys_ctx->addr_unit, 1);
 	ctx->pause = sys_ctx->pause;
-- 
2.47.3