mm/damon/core: prevent unnecessary overflow in damos_set_effective_quota()

On 32-bit systems, the throughput calculation in
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.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 1cd2430 ("mm/damon/schemes: implement time quota")
Signed-off-by: Quanmin Yan <[email protected]>
Reported-by: Andrew Paniakin <[email protected]>
Reviewed-by: SeongJae Park <[email protected]>
Cc: Kefeng Wang <[email protected]>
Cc: ze zuo <[email protected]>
Cc: <[email protected]>	[5.16+]
Signed-off-by: Andrew Morton <[email protected]>

Author: Quanmin Yan

mm/damon/core.c

@@ -2073,8 +2073,8 @@ 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);