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charset=UTF-8 Date: Tue, 18 Aug 2026 15:14:55 -0400 Message-Id: Cc: "Suren Baghdasaryan" , "Michal Hocko" , "Brendan Jackman" , "Johannes Weiner" , , , , To: "Qi Xi" , "Andrew Morton" , "Vlastimil Babka" From: "Zi Yan" Subject: Re: [PATCH] mm/page_isolation: fix UBSAN shift-out-of-bounds warning X-Mailer: aerc 0.21.0 References: <20260818112855.3831692-1-xiqi2@huawei.com> In-Reply-To: <20260818112855.3831692-1-xiqi2@huawei.com> X-ClientProxiedBy: CH0PR03CA0200.namprd03.prod.outlook.com (2603:10b6:610:e4::25) To IA0PR12MB8374.namprd12.prod.outlook.com (2603:10b6:208:40e::7) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: IA0PR12MB8374:EE_|SN7PR12MB7812:EE_ X-MS-Office365-Filtering-Correlation-Id: 303fa49d-6e4a-4038-a9d1-08defd5cfd85 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|23010399003|7416014|376014|1800799024|366016|6133799003|56012099006|10067099003|11063799006|22082099003|18002099003; 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Once the lock is > dropped, it scans the MAX_ORDER_NR_PAGES-aligned window [start_pfn, > boundary_pfn) locklessly, only to skip the free pages already handled > above and to detect in-use pages straddling the boundary. Since this > scan only reads page state to decide how far to skip and returns -EBUSY > on a straddling in-use page, it does not take the lock. > > The window also covers the adjacent pageblock, whose free pages stay on > the normal movable/CMA freelist and can be allocated concurrently. So > after the scan observes PageBuddy(page), another CPU can allocate the > page, leaving a stale value in page->private that makes "1 << order" shif= t > out of range. > > Use buddy_order_unsafe() to read the order exactly once (READ_ONCE), and > guard the shift with an order <=3D MAX_PAGE_ORDER check so it is never > performed with a bogus value. > > Fixes: b2c9e2fbba32 ("mm: make alloc_contig_range work at pageblock granu= larity") > Signed-off-by: Qi Xi > --- > mm/page_isolation.c | 13 ++++++++----- > 1 file changed, 8 insertions(+), 5 deletions(-) > The fix makes sense to me. Please Cc stable for this. Thanks. Sashiko has two more concerns: 1. order can still be bogus even if it is within MAX_PAGE_ORDER. a. it can trigger VM_WARN_ON_ONCE(); b. it can skip arbitrary pages and miss an in-use compound page. For 1a, we can remove VM_WARN_ON_ONCE() and just fail isolate_single_pageblock() if PageBuddy crosses the boundary, since pageblock_isolate_and_move_free_pages() handles it. For 1b, after we makes the change in 1a, PageBuddy skip cannot land us beyond the boundary, so the concern is gone. 2. In PageCompound() branch, a bogus nr_pages can also cause the shift-out-of-bounds. It will need a different fix. Hi Qi, Do you mind fixing 1a along with this patch? For this patch alone, Reviewed-by: Zi Yan --=20 Best Regards, Yan, Zi