[PATCH v4 2/2] mm/damon/reclaim: validate min_region_size to be power of 2

[Liew Rui Yan <aethernet65535@gmail.com>]

Problem
=======
When a user sets an invalid 'addr_unit' (e.g., 3) via
DAMON_RECLAIM, 'min_region_sz' becomes a non-power-of-2
value. This value eventually reaches damon_commit_ctx(), which does:

    dst->maybe_corrupted = true;
    if (!is_power_of_2(src->min_region_sz))
        return -EINVAL;

Although -EINVAL is returned, 'maybe_corrupted' is already set. The
running kdamond observers this flag and terminates unexpectedly.

"Unexpected termination" here means the kdamond exits without any user
request (e.g., not by writing 'N' to 'enabled').

User Impact
===========
Once kdamond terminates this way, it cannot be restarted via sysfs
because:

1. DAMON_RECLAIM is built into the kernel, so it cannot be unloaded and
   reloaded at runtime.
2. Writing 'N' to 'enabled' fails because kdamond no longer exists;
   Writing 'Y' does nothing, as 'enabled' is already Y.

Reproduction
============
1. Enable DAMON_RECLAIM
2. Set addr_unit=3
3. Commit inputs via 'commit_inputs'
4. Observe kdamond termination

Solution
========
Add an early validation in damon_reclaim_apply_parameters() to check
'min_region_sz' before any state change occurs. If it is non-power-of-2,
return -EINVAL immediately, preventing 'maybe_corrupted' from being set.

Fixes: 7db551fcfb2a ("mm/damon/reclaim: support addr_unit for DAMON_RECLAIM")
Cc: <stable@vger.kernel.org> # 6.18.x
Signed-off-by: Liew Rui Yan <aethernet65535@gmail.com>
---
 mm/damon/reclaim.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/damon/reclaim.c b/mm/damon/reclaim.c
index 86da14778658..2747eef5919d 100644
--- a/mm/damon/reclaim.c
+++ b/mm/damon/reclaim.c
@@ -204,6 +204,11 @@ static int damon_reclaim_apply_parameters(void)
 	param_ctx->addr_unit = addr_unit;
 	param_ctx->min_region_sz = max(DAMON_MIN_REGION_SZ / addr_unit, 1);
 
+	if (!is_power_of_2(param_ctx->min_region_sz)) {
+		err = -EINVAL;
+		goto out;
+	}
+
 	if (!damon_reclaim_mon_attrs.aggr_interval) {
 		err = -EINVAL;
 		goto out;
-- 
2.53.0