On 32-bit systems, the throughput calculation in function damos_set_effective_quota() is prone to unnecessary multiplication overflow. Using mult_frac() to fix it. Andrew Paniakin also recently found and privately reported this issue, on 64 bit systems. This can also happen on 64-bit systems, once the charged size exceeds ~17 TiB. On systems running for long time in production, this issue can actually happen. More specifically, when a DAMOS scheme having the time quota run for longtime, throughput calculation can overflow and set esz too small. As a result, speed of the scheme get unexpectedly slow. Fixes: 1cd243030059 ("mm/damon/schemes: implement time quota") Cc: # 5.16.x Signed-off-by: Quanmin Yan Reported-by: Andrew Paniakin Closes: N/A # privately reported --- mm/damon/core.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/mm/damon/core.c b/mm/damon/core.c index 831a9afc6cf6..fe1c19307ddd 100644 --- a/mm/damon/core.c +++ b/mm/damon/core.c @@ -2103,8 +2103,8 @@ static void damos_set_effective_quota(struct damos_quota *quota) if (quota->ms) { if (quota->total_charged_ns) - throughput = quota->total_charged_sz * 1000000 / - quota->total_charged_ns; + throughput = mult_frac(quota->total_charged_sz, 1000000, + quota->total_charged_ns); else throughput = PAGE_SIZE * 1024; esz = min(throughput * quota->ms, esz); -- 2.43.0