[PATCH v4 0/2] mm/damon: 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_LRU_SORT/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_LRU_SORT/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.

Solution
========
Add an early validation in damon_lru_sort_apply_parameters()
/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.

Patch 1 fixes the issue for DAMON_LRU_SORT.
Patch 2 fixes the issue for DAMON_RECLAIM.

Changes from v3
(https://lore.kernel.org/20260403052837.58063-1-aethernet65535@gmail.com)
- Improve commit message: clarify "unexpected termination".
- Add detailed User Impact with reason why kdamond cannot be restarted.
Changes from v2
(https://lore.kernel.org/20260402053756.26606-1-aethernet65535@gmail.com)
- Split the patch into two per-module patches.
- Add Fixes: and Cc: stable tags.
- Elaborate user impact and reproduction steps.
Changes from v1
(https://lore.kernel.org/20260331073231.30060-1-aethernet65535@gmail.com)
- Fix memory leak issue.
Changes from first attempt
(https://lore.kernel.org/20260327062627.66426-1-aethernet65535@gmail.com)
- Renamed the subject.
- Validate min_region_sz rather than addr_unit.

Liew Rui Yan (2):
  mm/damon/lru_sort: validate min_region_size to be power of 2
  mm/damon/reclaim: validate min_region_size to be power of 2

 mm/damon/lru_sort.c | 5 +++++
 mm/damon/reclaim.c  | 5 +++++
 2 files changed, 10 insertions(+)

-- 
2.53.0