[BUG] DAMON: 32-bit overflow selects wrong sampling interval

[Aveline Noir]

Hi,

In an unmodified i386 QEMU kernel built from mm-new commit
ad9f2d463666, an intervals goal selected a 2500 us sampling interval
after the first aggregation.  The same trace inputs give 2812 us with
full-width arithmetic.

The kernel has CONFIG_DAMON_VADDR, CONFIG_DAMON_SYSFS, and tracing
enabled.  The guest monitored a 1 MiB VADDR range as four contiguous
256 KiB regions, with sample_us=5000, aggr_us=100000, access_bp=400,
aggrs=1, and min_sample_us=2500.  The default minimum of 5000 us would
hide the difference.

At the first aggregation boundary, the four damos_before_apply records
for kdamond.0 showed nr_accesses=1,1,1,3.  The next
damon_monitor_intervals_tune record showed sample_us=2500; sysfs then
reported sample_us=2500 and aggr_us=50000.  The records cover the same
four 262144-byte regions before the first tune.

For that aggregation, the inputs to damon_get_intervals_score() are:

  max_access_events = 4 * 262144 * 20 = 20971520
  access_events     = (1 + 1 + 1 + 3) * 262144 = 1572864

On 32-bit unsigned long, max_access_events * access_bp wraps before
division by 10000.  This gives a target of 409364, a score of 38422 bp,
and the observed interval of 2500 us.  With full-width arithmetic, the
target is 838860, the score is 18750 bp, and the interval is 2812 us.
The later mult_frac() remainder multiplication does not overflow in the
observed 32-bit path, so the difference here comes from the target
multiplication.

The affected expression is in mm/damon/core.c.  Widening only the
target product would make the later 32-bit mult_frac() remainder
multiplication overflow for these same inputs.  The 2812 us result
requires sufficient width throughout the score calculation.

The 2500 us interval was measured on the unmodified kernel.  I
calculated 2812 us from the trace; I have not tested a patched kernel.
The i386 config, guest init, serial/trace log, and build/run steps:

  https://github.com/jm5905938/kernel-mail/tree/195d7017352a3d0f4a0475b98f3b03650f5c14ea/cases/2026/damon/2026-10-15-bug-reports/02-intervals-goal-32bit-overflow

Thanks,
Aveline Noir