Re: [PATCH mm-hotfixes-unstable v3] mm/damon/stat: avoid overflow in estimated_memory_bandwidth calculation

[sashiko-bot@kernel.org]

> From: Wang Guangke <gkwang@linx-info.com>
> 
> estimated_memory_bandwidth is calculated by access_bytes *
> USEC_PER_MSEC * MSEC_PER_SEC (= USEC_PER_SEC) / aggr_interval,
> and users read it via
> /sys/module/damon_stat/parameters/estimated_memory_bandwidth.
> 
> On 32-bit systems, access_bytes * USEC_PER_SEC overflows unsigned
> long once access_bytes exceeds about 4294 (U32_MAX / USEC_PER_SEC),
> so users get a wrong value.
> 
> Fix it by doing the calculation in 64-bit and splitting the division
> into quotient and remainder.
> 
> The issue was reproduced on a 32-bit system (qemu-system-i386): the
> parameter estimated_memory_bandwidth reported 362 instead of the
> expected 1926150; with the fix, the value matches the expectation.
> 
> Fixes: fabdd1e911da ("mm/damon/stat: calculate and expose estimated memory bandwidth")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Wang Guangke <gkwang@linx-info.com>
> Reviewed-by: SJ Park <sj@kernel.org>
> Signed-off-by: SJ Park <sj@kernel.org>

Sashiko has reviewed this patch and found no issues.

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