Re: [RFC PATCH 1/3] mm/damon/ops-common: avoid overflow in weighted hotness score

[sashiko-bot@kernel.org]

> damon_hot_score() combines the frequency and age subscores using the
> user supplied DAMOS quota weights.  The weights are unsigned int, and
> both the weighted sum and the total weight are calculated in unsigned
> int.  This is independent of the architecture word size.
> 
> A frequency weight of 50,000,000 makes the weighted sum wrap, so a
> region that is accessed in every sample gets a score of 13 instead of
> 99.  If the two weights add up to exactly 2^32, the total wraps to zero
> and the averaging division is skipped.
> 
> Accumulate both values in u64 and divide the weighted sum by the full
> total weight.  Add a KUnit case covering a large weight, weights summing
> above UINT_MAX, and zero weights.
> 
> The issue was discovered [1] by Sashiko.
> 
> [1] https://lore.kernel.org/20260623141527.044D11F000E9@smtp.kernel.org
> 
> Fixes: 198f0f4c58b9 ("mm/damon/vaddr,paddr: support pageout prioritization")
> Signed-off-by: DaeMyung Kang <charsyam@gmail.com>

Sashiko has reviewed this patch and found no issues.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261027135932.2301646-1-charsyam@gmail.com?part=1