Re: [PATCH v1 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.  Documentation/admin-guide/mm/damon/usage.rst
> instructs "echo $((1024*1024*1024)) > quotas/bytes", and 1 GiB * 10000 is
> 2500 * 2^32, so that documented value wraps to exactly zero; 256 MiB and
> every multiple of it do the same.  quota->esz then becomes zero while the
> goal is not achieved, the trailing "if (quota->sz && quota->sz < esz)" can
> only lower esz further, and damos_quota_is_full() is true on the first test
> of every charge window, so the scheme applies nothing and the goal is never
> approached.  Other sizes are wrong without being zero: 500000 yields 70503.
> 
> Saturate to ULONG_MAX, which is what the same function already writes for a
> scheme with no size quota.  Widening esz_bp instead would reach the consist
> tuner, which runs the same field through damon_feed_loop_next_input(),
> unsigned long in and out; bounding the multiply keeps the change to this
> branch.  On 32-bit a large size quota then behaves like no size quota
> rather than like a dead scheme.
> 
> 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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