Re: [PATCH v4 0/2] mm/damon: validate min_region_size to be power of 2

[SeongJae Park <sj@kernel.org>]

On Fri, 10 Apr 2026 12:42:57 +0800 Liew Rui Yan <aethernet65535@gmail.com> wrote:

> 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.

Thank you for making the change.  Looks good to me.  I gave my 'Reviewed-by:'
for the two patches as replies.

Andrew, this series is for hotfixes that deserve Cc-ing stable@.  Could you
please pick this up into mm.git?  I don't think these are super-urgent and I
have some capacities to manage this myself, though.  So if you prefer to, I can
stash this in my tree for now and repost after next rc1.


Thanks,
SJ

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