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

[SeongJae Park <sj@kernel.org>]

On Fri, 10 Apr 2026 12:42:59 +0800 Liew Rui Yan <aethernet65535@gmail.com> wrote:

> 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>

Reviewed-by: SeongJae Park <sj@kernel.org>


Thanks,
SJ

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