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Sat, 19 Sep 2026 19:31:18 -0700 (PDT) From: Donggeun Yoo To: SJ Park , Andrew Morton Cc: damon@lists.linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, donggeunyoo.kernel@gmail.com, stable@vger.kernel.org Subject: [PATCH v2 1/3] mm/damon/core: prevent size quota overflow in the temporal goal tuner Date: Sun, 20 Sep 2026 11:31:09 +0900 Message-ID: <20260920023111.2466265-2-donggeunyoo.kernel@gmail.com> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260920023111.2466265-1-donggeunyoo.kernel@gmail.com> References: <20260920023111.2466265-1-donggeunyoo.kernel@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" damos_goal_tune_esz_bp_temporal() converts the scheme's size quota into basis points with "quota->esz_bp =3D quota->sz * 10000", both unsigned long, and damos_set_effective_quota() divides the result back by 10000. quotas/bytes is unbounded; bytes_store() hands it to kstrtoul() as is. On 32-bit the product wraps for any size quota above ULONG_MAX / 10000, that is 429496 bytes. Documentation/admin-guide/mm/damon/usage.rst instructs "echo $((1024*1024*1024)) > quotas/bytes", and 1 GiB * 10000 is 2500 * 2^32, so that documented value wraps to exactly zero; 256 MiB and every multiple of it do the same. quota->esz then becomes zero while the goal is not achieved, the trailing "if (quota->sz && quota->sz < esz)" can only lower esz further, and damos_quota_is_full() is true on the first test of every charge window, so the scheme applies nothing and the goal is never approached. Other sizes are wrong without being zero: 500000 yields 70503. While the addr_unit parameter effectively mitigates the overflow risk by scaling down the values written to quotas/bytes, it does not fundamentally solve the issue. Theoretically, an overflow can still occur if the scaled value is exceptionally large. Furthermore, because addr_unit is exclusive to the paddr operations set, vaddr and fvaddr contexts remain fully exposed to this overflow since they take unscaled raw byte values. The 64-bit boundary is reachable without any scaling: bytes_store() takes whatever kstrtoul() parses, so a quotas/bytes above 1844674407370955 wraps the multiply there too. Bound the conversion, so a size quota it cannot represent falls to the ULONG_MAX the function already writes for a scheme with no size quota. Widening esz_bp instead would reach the consist tuner, which runs the same field through damon_feed_loop_next_input(), unsigned long in and out. On 32-bit a large size quota then behaves like no size quota rather than like a dead scheme. Fixes: af738a6a00c1 ("mm/damon/core: introduce DAMOS_QUOTA_GOAL_TUNER_TEMPO= RAL") Cc: # 7.1.x Signed-off-by: Donggeun Yoo --- Measured on i386 under QEMU: one paddr context with a stat scheme, the temporal goal tuner, and one unachieved user_input goal. Each size is written to quotas/bytes, the kdamond is started, and quotas/effective_bytes is read back after update_schemes_effective_quotas. quotas/bytes effective_bytes effective_bytes before after 4096 4096 4096 429496 429496 429496 429497 0 429496 268435456 0 429496 1073741824 0 429496 500000 70503 429496 4294967295 429495 429496 0 429496 429496 Everything the conversion can hold is unchanged, and 429496 is what the no-size-quota row already produced before the patch. Patch 2 pins the same boundary at ULONG_MAX / 10000 and so runs on any word size. Without this patch it fails on x86_64: # damos_test_esz_goal_temporal: EXPECTATION FAILED at mm/damon/tests/core= -kunit.h:1970 Expected s->quota.esz =3D=3D (~0UL) / 10000, but s->quota.esz =3D=3D 0 (0x0) # damos_test_esz_goal_temporal: EXPECTATION FAILED at mm/damon/tests/core= -kunit.h:1974 Expected s->quota.esz =3D=3D (~0UL) / 10000, but s->quota.esz =3D=3D 1844674407370954 (0x68db8bac710ca) mm/damon/core.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/mm/damon/core.c b/mm/damon/core.c index 2258b72da7a78..16d4145379a2b 100644 --- a/mm/damon/core.c +++ b/mm/damon/core.c @@ -3274,7 +3274,7 @@ static void damos_goal_tune_esz_bp_temporal(struct da= mon_ctx *c, =20 if (score >=3D 10000) quota->esz_bp =3D 0; - else if (quota->sz) + else if (quota->sz && quota->sz <=3D ULONG_MAX / 10000) quota->esz_bp =3D quota->sz * 10000; else quota->esz_bp =3D ULONG_MAX; --=20 2.53.0 From nobody Thu Sep 24 21:18:04 2026 Received: from mail-pz2-f43.google.com (mail-pz2-f43.google.com [74.125.228.43]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id BB2DB2BE034 for ; 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charset="utf-8" damos_goal_tune_esz_bp_temporal() encodes the size quota in basis points, so the conversion is exact only up to ULONG_MAX / 10000. Pin the three sizes around that boundary: the largest one that fits, the first one that does not, and ULONG_MAX. Signed-off-by: Donggeun Yoo Reviewed-by: SJ Park --- mm/damon/tests/core-kunit.h | 48 +++++++++++++++++++++++++++++++++++++ 1 file changed, 48 insertions(+) diff --git a/mm/damon/tests/core-kunit.h b/mm/damon/tests/core-kunit.h index 5ff0436c58441..a0604788bc638 100644 --- a/mm/damon/tests/core-kunit.h +++ b/mm/damon/tests/core-kunit.h @@ -1929,6 +1929,53 @@ static void damon_test_rand(struct kunit *test) } } =20 +static void damos_test_esz_goal_temporal(struct kunit *test) +{ + struct damos_access_pattern pattern =3D {}; + struct damos_watermarks wmarks =3D {}; + struct damos_quota quota =3D { + .goal_tuner =3D DAMOS_QUOTA_GOAL_TUNER_TEMPORAL, + }; + struct damos_quota_goal *goal; + struct damon_ctx *ctx; + struct damos *s; + + ctx =3D damon_new_ctx(); + KUNIT_ASSERT_NOT_NULL(test, ctx); + + s =3D damon_new_scheme(&pattern, DAMOS_STAT, 0, "a, &wmarks, + NUMA_NO_NODE); + if (!s) { + damon_destroy_ctx(ctx); + kunit_skip(test, "scheme alloc fail"); + } + damon_add_scheme(ctx, s); + + goal =3D damos_new_quota_goal(DAMOS_QUOTA_USER_INPUT, 10000); + if (!goal) { + damon_destroy_ctx(ctx); + kunit_skip(test, "quota goal alloc fail"); + } + goal->current_value =3D 0; + damos_add_quota_goal(&s->quota, goal); + + /* The largest size quota the basis-point conversion can hold. */ + s->quota.sz =3D ULONG_MAX / 10000; + damos_set_effective_quota(ctx, s); + KUNIT_EXPECT_EQ(test, s->quota.esz, ULONG_MAX / 10000); + + /* Any larger one saturates instead of wrapping. */ + s->quota.sz =3D ULONG_MAX / 10000 + 1; + damos_set_effective_quota(ctx, s); + KUNIT_EXPECT_EQ(test, s->quota.esz, ULONG_MAX / 10000); + + s->quota.sz =3D ULONG_MAX; + damos_set_effective_quota(ctx, s); + KUNIT_EXPECT_EQ(test, s->quota.esz, ULONG_MAX / 10000); + + damon_destroy_ctx(ctx); +} + static struct kunit_case damon_test_cases[] =3D { KUNIT_CASE(damon_test_target), KUNIT_CASE(damon_test_regions), @@ -1965,6 +2012,7 @@ static struct kunit_case damon_test_cases[] =3D { KUNIT_CASE(damon_test_is_last_region), KUNIT_CASE(damon_test_walk_control_obsolete), KUNIT_CASE(damon_test_rand), + KUNIT_CASE(damos_test_esz_goal_temporal), {}, }; 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Sat, 19 Sep 2026 19:31:26 -0700 (PDT) From: Donggeun Yoo To: SJ Park , Andrew Morton Cc: damon@lists.linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, donggeunyoo.kernel@gmail.com, David Hildenbrand , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jonathan Corbet , Shuah Khan , Randy Dunlap , linux-doc@vger.kernel.org Subject: [PATCH v2 3/3] Docs/admin-guide/mm/damon/usage: document the temporal tuner's quota limit Date: Sun, 20 Sep 2026 11:31:11 +0900 Message-ID: <20260920023111.2466265-4-donggeunyoo.kernel@gmail.com> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260920023111.2466265-1-donggeunyoo.kernel@gmail.com> References: <20260920023111.2466265-1-donggeunyoo.kernel@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" The temporal goal tuner converts the size quota into basis points in an unsigned long, so a quotas/bytes above ULONG_MAX / 10000 does not fit, and is handled as if no size quota were set. On 32-bit that is 429496 bytes, well below the "echo $((1024*1024*1024)) > quotas/bytes" this document instructs. addr_unit does not raise the limit, though a larger unit does make it harder to reach. Note both in the goal_tuner description. Signed-off-by: Donggeun Yoo --- Documentation/admin-guide/mm/damon/usage.rst | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/Documentation/admin-guide/mm/damon/usage.rst b/Documentation/a= dmin-guide/mm/damon/usage.rst index ba47255448564..ea7443b307826 100644 --- a/Documentation/admin-guide/mm/damon/usage.rst +++ b/Documentation/admin-guide/mm/damon/usage.rst @@ -455,6 +455,13 @@ the background design of the feature and the name of t= he selectable algorithms. Refer to :ref:`goals directory ` for the goals setup. =20 +The ``temporal`` algorithm converts the size quota into basis points, so it +cannot represent a ``bytes`` value larger than 429496 on 32-bit kernels, or +1844674407370955 on 64-bit ones. Larger values are handled as if no size +quota were set. :ref:`addr_unit ` does not raise = the +limit, but it does make the limit harder to reach: with a larger unit, the +same amount of memory is written to ``bytes`` as a smaller number. + You can set the action-failed memory quota charging ratio by writing the numerator and the denominator for the ratio to ``fail_charge_num`` and ``fail_charge_denom`` files, respectively. Reading those files will retur= n the --=20 2.53.0