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[98.38.17.99]) by smtp.googlemail.com with ESMTPSA id 5614622812f47-466df93ce67sm4529016b6e.1.2026.03.08.15.35.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 08 Mar 2026 15:35:51 -0700 (PDT) From: Jim Cromie To: linux-kernel@vger.kernel.org Cc: Jim Cromie , Matthew Auld , Arun Pravin , Christian Koenig , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Simona Vetter , Jani Nikula , Luca Coelho , dri-devel@lists.freedesktop.org Subject: [PATCH 1/2] drm_buddy: fix power-of-2 rounding errs Date: Sun, 8 Mar 2026 16:35:36 -0600 Message-ID: <20260308223538.96729-2-jim.cromie@gmail.com> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260308223538.96729-1-jim.cromie@gmail.com> References: <20260308223538.96729-1-jim.cromie@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" On DRM-CI, I encountered this kunit:arm32 failure. [23:19:40] [PASSED] drm_test_buddy_alloc_clear [23:19:40] [PASSED] drm_test_buddy_alloc_range_bias [23:19:41] [PASSED] drm_test_buddy_fragmentation_performance [23:19:41] # drm_test_buddy_alloc_exceeds_max_order: EXPECTATION FAILED= at drivers/gpu/drm/tests/drm_buddy_test.c:889 [23:19:41] Expected err =3D=3D -22, but [23:19:41] err =3D=3D 0 (0x0) [23:19:41] ------------[ cut here ]------------ [23:19:41] WARNING: drivers/gpu/drm/drm_buddy.c:405 at drm_buddy_fini+0x114= /0x1b8, CPU#0: kunit_try_catch/74 [23:19:41] CPU: 0 UID: 0 PID: 74 Comm: kunit_try_catch Tainted: G = N 7.0.0-rc1-gdfb0bcedd08a #1 VOLUNTARY [23:19:41] Tainted: [N]=3DTEST [23:19:41] Hardware name: Generic DT based system [23:19:41] Call trace: [23:19:41] unwind_backtrace from show_stack+0x10/0x14 [23:19:41] show_stack from dump_stack_lvl+0x3c/0x4c [23:19:41] dump_stack_lvl from __warn+0xe8/0x1c4 [23:19:41] __warn from warn_slowpath_fmt+0xa4/0xc0 [23:19:41] warn_slowpath_fmt from drm_buddy_fini+0x114/0x1b8 [23:19:41] drm_buddy_fini from drm_test_buddy_alloc_exceeds_max_order+0x1c= 8/0x36c [23:19:41] drm_test_buddy_alloc_exceeds_max_order from kunit_try_run_case+= 0x78/0x1c8 [23:19:41] kunit_try_run_case from kunit_generic_run_threadfn_adapter+0x1c= /0x34 [23:19:41] kunit_generic_run_threadfn_adapter from kthread+0x108/0x134 [23:19:41] kthread from ret_from_fork+0x14/0x28 [23:19:41] Exception stack(0xf0bd5fb0 to 0xf0bd5ff8) [23:19:41] 5fa0: 00000000 00000000 0000= 0000 00000000 [23:19:41] 5fc0: 00000000 00000000 00000000 00000000 00000000 00000000 0000= 0000 00000000 [23:19:41] 5fe0: 00000000 00000000 00000000 00000000 00000013 00000000 [23:19:41] ---[ end trace 0000000000000000 ]--- [23:19:41] # drm_test_buddy_alloc_exceeds_max_order: drivers/gpu/drm/dr= m_buddy.c:406: buddy_fini() root [23:19:41] ------------[ cut here ]------------ [23:19:41] WARNING: drivers/gpu/drm/drm_buddy.c:414 at drm_buddy_fini+0x1b4= /0x1b8, CPU#0: kunit_try_catch/74 [23:19:41] CPU: 0 UID: 0 PID: 74 Comm: kunit_try_catch Tainted: G W = N 7.0.0-rc1-gdfb0bcedd08a #1 VOLUNTARY [23:19:41] Tainted: [W]=3DWARN, [N]=3DTEST [23:19:41] Hardware name: Generic DT based system [23:19:41] Call trace: [23:19:41] unwind_backtrace from show_stack+0x10/0x14 [23:19:41] show_stack from dump_stack_lvl+0x3c/0x4c [23:19:41] dump_stack_lvl from __warn+0xe8/0x1c4 [23:19:41] __warn from warn_slowpath_fmt+0xa4/0xc0 [23:19:41] warn_slowpath_fmt from drm_buddy_fini+0x1b4/0x1b8 [23:19:41] drm_buddy_fini from drm_test_buddy_alloc_exceeds_max_order+0x1c= 8/0x36c [23:19:41] drm_test_buddy_alloc_exceeds_max_order from kunit_try_run_case+= 0x78/0x1c8 [23:19:41] kunit_try_run_case from kunit_generic_run_threadfn_adapter+0x1c= /0x34 [23:19:41] kunit_generic_run_threadfn_adapter from kthread+0x108/0x134 [23:19:41] kthread from ret_from_fork+0x14/0x28 [23:19:41] Exception stack(0xf0bd5fb0 to 0xf0bd5ff8) [23:19:41] 5fa0: 00000000 00000000 0000= 0000 00000000 [23:19:41] 5fc0: 00000000 00000000 00000000 00000000 00000000 00000000 0000= 0000 00000000 [23:19:41] 5fe0: 00000000 00000000 00000000 00000000 00000013 00000000 [23:19:41] ---[ end trace 0000000000000000 ]--- [23:19:41] [FAILED] drm_test_buddy_alloc_exceeds_max_order [23:19:41] # drm_buddy: Testing DRM buddy manager, with random_seed=3D0= xacce106c [23:19:41] # module: drm_buddy_test [23:19:41] # drm_buddy: pass:8 fail:1 skip:0 total:9 This error in drm_test_buddy_alloc_exceeds_max_order (Expected -22, got 0) is caused by a 64-bit truncation bug in the DRM buddy allocator's core logic. On 32-bit architectures (like arm32), the standard roundup_pow_of_two() and rounddown_pow_of_two() macros use unsigned long internally. When these are called with u64 memory sizes (as drm_buddy.c does), any size above 4GB is truncated. The test tries to allocate ~9GB on a ~10GB manager. Due to truncation, it actually asks for ~1GB on a ~2GB manager, which unexpectedly succeeds. This then causes a mm->avail mismatch warning when the test finishes. Use 64-bit safe power-of-two calculations. Since ilog2() is already safe for u64, I'll use it to implement safe rounding. Signed-off-by: Jim Cromie --- drivers/gpu/drm/drm_buddy.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/drivers/gpu/drm/drm_buddy.c b/drivers/gpu/drm/drm_buddy.c index dbf984f8e301..8f23fb615d47 100644 --- a/drivers/gpu/drm/drm_buddy.c +++ b/drivers/gpu/drm/drm_buddy.c @@ -919,7 +919,7 @@ static int __alloc_contig_try_harder(struct drm_buddy *= mm, u64 modify_size; int err; =20 - modify_size =3D rounddown_pow_of_two(size); + modify_size =3D 1ULL << ilog2(size); pages =3D modify_size >> ilog2(mm->chunk_size); order =3D fls(pages) - 1; if (order =3D=3D 0) @@ -1140,7 +1140,7 @@ int drm_buddy_alloc_blocks(struct drm_buddy *mm, =20 /* Roundup the size to power of 2 */ if (flags & DRM_BUDDY_CONTIGUOUS_ALLOCATION) { - size =3D roundup_pow_of_two(size); + size =3D 1ULL << (ilog2(size - 1) + 1); min_block_size =3D size; /* Align size value to min_block_size */ } else if (!IS_ALIGNED(size, min_block_size)) { --=20 2.53.0