mm/damon/core: charge only the part of a region the filter left

Patch series "mm/damon/core: fix the size charged for a filter-trimmed
region", v2.

An address range DAMOS filter that partially overlaps a monitoring region
splits it, so the scheme action reaches only one side of the boundary. 
damos_apply_scheme() reads the region size before that split and never
re-reads it, so the quota charge and schemes/<S>/stats/sz_tried account
for the whole original region.

Patch 1 re-reads the size after the core filters have run.  Patch 2 adds
KUnit coverage for what the function charges and reports, for the trimmed
cases and for the cases that must not change.


This patch (of 2):

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.

User impact is that DAMOS could unexpectedly slowly run, due to
over-charged quota.  DAMOS stat could also be confusing.  No critical
events such as crashes or leask happen.

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

Link: https://lore.kernel.org/20260921152443.80132-1-sj@kernel.org
Link: https://lore.kernel.org/20260921152443.80132-2-sj@kernel.org
Fixes: ab9bda001b68 ("mm/damon/core: introduce address range type damos filter")
Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@gmail.com>
Signed-off-by: SJ Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: SJ Park <sj@kernel.org>
Cc: Brendan Higgins <brendan.higgins@linux.dev>
Cc: David Gow <david@davidgow.net>
Cc: <stable@vger.kernel.org>

0 files changed, 0 insertions, 0 deletions