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Tue, 26 Oct 2021 09:53:33 +0000 Received: from PH7PR11MB5819.namprd11.prod.outlook.com ([fe80::3508:ff4c:362d:579c]) by PH7PR11MB5819.namprd11.prod.outlook.com ([fe80::3508:ff4c:362d:579c%7]) with mapi id 15.20.4628.020; Tue, 26 Oct 2021 09:53:33 +0000 Subject: Re: [PATCH] ARM: add BUILD_BUG_ON to check if fixmap range spans multiple pmds To: "Russell King (Oracle)" , Linus Walleij Cc: Thomas Gleixner , Ard Biesheuvel , Linux ARM , linux-kernel References: <20211020054942.1608637-1-quanyang.wang@windriver.com> From: Quanyang Wang Message-ID: Date: Tue, 26 Oct 2021 17:53:25 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.13.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-ClientProxiedBy: HK2PR02CA0215.apcprd02.prod.outlook.com (2603:1096:201:20::27) To PH7PR11MB5819.namprd11.prod.outlook.com (2603:10b6:510:13b::9) MIME-Version: 1.0 Received: from [128.224.162.199] (60.247.85.82) by HK2PR02CA0215.apcprd02.prod.outlook.com (2603:1096:201:20::27) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4628.15 via Frontend Transport; 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On 10/26/21 4:59 PM, Russell King (Oracle) wrote: > On Sun, Oct 24, 2021 at 11:44:31PM +0200, Linus Walleij wrote: >> On Wed, Oct 20, 2021 at 7:50 AM wrote: >> >>> From: Quanyang Wang >>> >>> Not only the early fixmap range, but also the fixmap range should be >>> checked if it spans multiple pmds. When enabling CONFIG_DEBUG_HIGHMEM, >>> some systems which contain up to 16 CPUs will crash. >>> >>> Signed-off-by: Quanyang Wang >> >> Looks reasonable to me. >> Reviewed-by: Linus Walleij >> >> Please submit this patch into Russell's patch tracker. > > ... and has totally broken what looks like _all_ ARM kernel builds. This patch is intended to trigger build error when it check the value of __end_of_fixmap_region is equal or larger than 256. In fact, it breaks the ARM kernel builds which NR_CPUS is equal or more than 16. If CONFIG_DEBUG_HIGHMEM is enabled, all ARM builds which NR_CPUS is more than 8 will fail. It > can not have been tested. I tested this patch with allyesconfig instead of some configs in arch/arm/configs/. In allyesconfig, NR_CPUS is 4, so it not trigger build error. Then I changed it to 8 to verify my patch. Maybe it's uncovered a previously unknown > problem, Yes, at my side, axm5516 with CONFIG_DEBUG_HIGHMEM always falls into crash. Other ARM platform which contains more than 8 CPUs may encounter the same issue. but causing such a wide-range regression is disappointing. Sorry for not consider this thoroughly. Thanks, Quanyang > I'm going to revert this commit. >