[PATCH v2 0/3] mm/damon: fix the temporal goal tuner's size quota conversion

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

damos_goal_tune_esz_bp_temporal() hands the size quota to
damos_set_effective_quota() through quota->esz_bp in basis points, and the
multiply that gets it there is unchecked.  On 32-bit it wraps from 429497
bytes on, and wraps to exactly zero at the 1 GiB that
Documentation/admin-guide/mm/damon/usage.rst uses as its example, since
1 GiB * 10000 is 2500 * 2^32.  A zero effective quota makes
damos_quota_is_full() true on the first test of every charge window, so the
scheme applies nothing for as long as the goal is unachieved.

Patch 1 bounds the conversion.  Patch 2 pins the boundary in the core kunit
suite, where it fails without patch 1 on any word size.  Patch 3 documents
the ceiling that remains once the conversion is bounded: the basis-point
form cannot represent a size quota above ULONG_MAX / 10000, so a larger one
is handled as if no size quota were set.

Changes since v1
(https://lore.kernel.org/damon/20260919071324.1583280-1-donggeunyoo.kernel@gmail.com/):
- patch 1: bound the existing multiply arm rather than reorder the two
  arms, per SJ Park's review.  Same behavior, one changed line.
- patch 1: address addr_unit -- it mitigates but does not solve this, and
  the 64-bit boundary is reachable without any scaling.
- patch 2: build the scheme and the goal with damon_new_scheme(),
  damos_new_quota_goal() and damos_add_quota_goal() instead of a stack
  damos and an open-coded list_add(); drop the max_sz local.
- patch 3: new, also suggested by SJ Park.

DAMON kunit: 36 tests, all passing with both code patches; 35 passing and
damos_test_esz_goal_temporal failing with patch 2 alone.  DAMON selftests:
15 of 15 pass unpatched and patched.  They do not reach the changed
function, though.  A build with a print at its entry stayed silent through
all fifteen and fired as soon as a scheme was configured with the temporal
tuner.

Donggeun Yoo (3):
  mm/damon/core: prevent size quota overflow in the temporal goal tuner
  mm/damon/tests/core-kunit: test the temporal tuner's size quota
    conversion
  Docs/admin-guide/mm/damon/usage: document the temporal tuner's quota
    limit

 Documentation/admin-guide/mm/damon/usage.rst |  7 +++
 mm/damon/core.c                              |  2 +-
 mm/damon/tests/core-kunit.h                  | 48 ++++++++++++++++++++
 3 files changed, 56 insertions(+), 1 deletion(-)


base-commit: 498ee28e5ec4727f829507c4a1bde3ab1b7704cd
-- 
2.53.0