[PATCH v1 1/2] mm/damon/core: charge only the part of a region the filter left

[Donggeun Yoo <donggeunyoo.kernel@gmail.com>]

An address range DAMOS filter that partially overlaps a monitoring
region splits the region at the filter boundary, so the scheme action is
applied to only one side of it.  damos_apply_scheme() reads the region
size once on entry, before damos_core_filter_out() performs that split.

The quota charge and the statistics update therefore account for the
region as it was before the trim.  schemes/<S>/stats/sz_tried counts
memory the filter excluded, which Documentation/mm/damon/design.rst says
is not counted as tried, and with quotas/bytes set the excluded part is
charged against the budget, throttling the scheme to a fraction of what
was configured.

Re-read the region size after the core filters have run.

Fixes: ab9bda001b68 ("mm/damon/core: introduce address range type damos filter")
Cc: <stable@vger.kernel.org> # 6.6.x
Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@gmail.com>
---
KUnit, mm/damon/tests, x86_64, with patch 2 applied to both trees:

  without patch 1  36/38  damos_test_apply_scheme_filtered_sz and
                          damos_test_apply_scheme_filter_sz_unchanged fail
  with patch 1     38/38

  Expected s->stat.sz_tried == 2 * min_sz, but
      s->stat.sz_tried == 16384 (0x4000)
      2 * min_sz == 8192 (0x2000)

All 15 DAMON selftests pass with the series applied, in a QEMU guest booted
with slab_nomerge and CONFIG_DEBUG_INFO, the latter so drgn could run two of
them.  sysfs_update_schemes_tried_regions_wss_estimation.py and
damos_quota.py are timing sensitive and have each failed intermittently on
both patched and unpatched trees over 15 runs.

One path is not covered by patch 2: the quota split failing at
damon_split_region_at().  A KUnit case cannot make that allocation fail.  The
sibling exit, a quota remainder below one region, is covered and reaches the
same update_stat label.

 mm/damon/core.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index dd4317df42ff..68f1749ded43 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2784,6 +2784,7 @@ static void damos_apply_scheme(struct damon_ctx *c, struct damon_target *t,
 		}
 		if (damos_core_filter_out(c, t, r, s))
 			return;
+		sz = damon_sz_region(r);
 		ktime_get_coarse_ts64(&begin);
 		trace_damos_before_apply(cidx, sidx, tidx, r, nr_accesses,
 				damon_nr_regions(t), do_trace);
-- 
2.53.0