[PATCH v1.1 5/6] mm/damon/sysfs: read addr_unit only once in damon_sysfs_apply_inputs()

damon_sysfs_apply_inputs() reads addr_unit twice.  It could race with
addr_unit_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, the DAMON core layer handles the wrong
min_region_sz (!is_power_of_2()).  Even if somehow the race ended up
making a min_region_sz that is different from the user's intention but
still valid, 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]

Fixes: 540a2aebc657 ("mm/damon/sysfs: implement addr_unit file under context dir")
Cc: <[email protected]> # 6.18.x
Signed-off-by: SJ Park <[email protected]>
---
 mm/damon/sysfs.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
index b5fe036f78015..65a502c7746c0 100644
--- a/mm/damon/sysfs.c
+++ b/mm/damon/sysfs.c
@@ -2099,11 +2099,11 @@ static int damon_sysfs_apply_inputs(struct damon_ctx *ctx,
 	err = damon_select_ops(ctx, sys_ctx->ops_id);
 	if (err)
 		return err;
-	ctx->addr_unit = sys_ctx->addr_unit;
+	ctx->addr_unit = READ_ONCE(sys_ctx->addr_unit);
 	/* addr_unit is respected by only DAMON_OPS_PADDR */
 	if (sys_ctx->ops_id == DAMON_OPS_PADDR)
 		ctx->min_region_sz = max(
-				DAMON_MIN_REGION_SZ / sys_ctx->addr_unit, 1);
+				DAMON_MIN_REGION_SZ / ctx->addr_unit, 1);
 	ctx->pause = sys_ctx->pause;
 	err = damon_sysfs_set_attrs(ctx, sys_ctx->attrs);
 	if (err)
-- 
2.47.3