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[PATCH] mm/percpu-km: fix bitmap overflow and accounting in pcpu_create_chunk() Cc: "Christoph Lameter" , , , "Sashiko" , "Zi Yan" To: "Dennis Zhou" , "Tejun Heo" , "Christoph Lameter" , "Andrew Morton" From: "Zi Yan" X-Mailer: aerc 0.21.0 References: <20260709-fix-pcpu_create_chunk-in-percpu-km-v1-1-1f64745a84cc@nvidia.com> In-Reply-To: <20260709-fix-pcpu_create_chunk-in-percpu-km-v1-1-1f64745a84cc@nvidia.com> X-ClientProxiedBy: BN1PR14CA0016.namprd14.prod.outlook.com (2603:10b6:408:e3::21) To IA0PR12MB8374.namprd12.prod.outlook.com (2603:10b6:208:40e::7) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: IA0PR12MB8374:EE_|LVXPR12MB999196:EE_ X-MS-Office365-Filtering-Correlation-Id: 8b33bd20-f8c9-471d-b95d-08dee693f76f X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|23010399003|1800799024|366016|11063799006|56012099006|22082099003|18002099003|10067099003; X-Microsoft-Antispam-Message-Info: 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu Jul 9, 2026 at 3:12 PM EDT, Zi Yan wrote: > In pcpu_create_chunk(), nr_pages is the total contiguous backing > allocation, i.e., nr_units * pcpu_unit_pages, but pcpu_chunk_populated() > uses it to set chunk->populated, whose size is pcpu_unit_pages, bitmap. > Since bit N in chunk->populated means page offset N inside every unit is > backed. When nr_units > 1, the function writes beyond chunk->populated. F= ix > it by using chunk->nr_pages. > > It also fixes the global pcpu_nr_empty_pop_pages accounting, since > pcpu_balance_free() only iterates up to chunk->nr_pages. > > Commit a63d4ac4ab609 ("percpu: make percpu-km set chunk->populated bitmap > properly") introduced the bitmap overflow issue. Later, > commit b539b87fed37f ("percpu: implmeent pcpu_nr_empty_pop_pages and > chunk->nr_populated") added pcpu_nr_empty_pop_pages and caused the > accounting issue. A gentle ping on this patch. It would be great to get some feedback. Thanks. > > Fixes: a63d4ac4ab609 ("percpu: make percpu-km set chunk->populated bitmap= properly") > Reported-by: Sashiko > Closes: https://sashiko.dev/#/patchset/20260703-keep-subpage-private-zero= -at-free-v2-0-2970fe777dd6%40nvidia.com?part=3D1 > Assisted-by: Codex:GPT-5 > Signed-off-by: Zi Yan > --- > To make nr_units > 1 in percpu-km, we need SMP && !MMU && NEED_PER_CPU_KM= . > RISC-V can have SMP && !MMU. SH J2 can have it too. The kconfig analysis = is > done by Codex. > --- > mm/percpu-km.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/percpu-km.c b/mm/percpu-km.c > index 7ffe84adadb9d..65fd5580e4472 100644 > --- a/mm/percpu-km.c > +++ b/mm/percpu-km.c > @@ -75,7 +75,7 @@ static struct pcpu_chunk *pcpu_create_chunk(gfp_t gfp) > chunk->base_addr =3D page_address(pages); > =20 > spin_lock_irqsave(&pcpu_lock, flags); > - pcpu_chunk_populated(chunk, 0, nr_pages); > + pcpu_chunk_populated(chunk, 0, chunk->nr_pages); > spin_unlock_irqrestore(&pcpu_lock, flags); > =20 > pcpu_stats_chunk_alloc(); > > --- > base-commit: 59e34180e138a293260301c139cf7ee800978533 > change-id: 20260709-fix-pcpu_create_chunk-in-percpu-km-6dfa8f28b175 > > Best regards, --=20 Best Regards, Yan, Zi