Re: [PATCH v3 1/2] mm/damon/core: prevent size quota overflow in the temporal goal tuner

[sashiko-bot@kernel.org]

> damos_goal_tune_esz_bp_temporal() converts the scheme's size quota
> into basis points with "quota->esz_bp = quota->sz * 10000", both
> unsigned long, and damos_set_effective_quota() divides the result
> back by 10000.  quotas/bytes is unbounded; bytes_store() hands it to
> kstrtoul() as is.
> 
> On 32-bit the product wraps for any size quota above ULONG_MAX /
> 10000, that is 429496 bytes.  It lands on exactly zero when the quota
> is a multiple of 256 MiB, which includes the 1 GiB that
> Documentation/admin-guide/mm/damon/usage.rst uses in its example, and
> a zero effective quota makes damos_quota_is_full() true on the first
> test of every charge window.  The scheme then applies nothing while
> the goal is unachieved.  Other wrapped values are simply wrong, and
> any product below 10000 divides to a zero effective quota too: 429497
> gives 0, 500000 gives 70503.
> [ ... ]
> effective quota becomes ULONG_MAX / 10000 instead of a wrapped value.
> 
> Fixes: af738a6a00c1 ("mm/damon/core: introduce DAMOS_QUOTA_GOAL_TUNER_TEMPORAL")
> Cc: <stable@vger.kernel.org> # 7.1.x
> Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@gmail.com>

Sashiko has reviewed this patch and found no issues. It looks great!

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