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X-CSE-ConnectionGUID: mx6iYmbhQoG0FLrcuu1qkw== X-CSE-MsgGUID: Bj47Yn0DTVGZ+oCpjiqJCw== X-IronPort-AV: E=McAfee;i="6800,10657,11529"; a="61840234" X-IronPort-AV: E=Sophos;i="6.17,312,1747724400"; d="scan'208";a="61840234" Received: from orviesa005.jf.intel.com ([10.64.159.145]) by orvoesa107.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 22 Aug 2025 10:08:32 -0700 X-CSE-ConnectionGUID: 5Nu5hnUPTz+8m2WU1ivarg== X-CSE-MsgGUID: YdsZ6B6rSVa6oYkMnHlSKw== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.17,312,1747724400"; d="scan'208";a="174065694" Received: from tfalcon-desk.amr.corp.intel.com (HELO [10.125.108.168]) ([10.125.108.168]) by orviesa005-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 22 Aug 2025 10:08:31 -0700 Message-ID: <79027c6f-f2f3-41b2-9ff3-c5576fc06c5c@intel.com> Date: Fri, 22 Aug 2025 10:08:30 -0700 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm: fix KASAN build error due to p*d_populate_kernel() To: Harry Yoo Cc: Liam.Howlett@oracle.com, akpm@linux-foundation.org, andreyknvl@gmail.com, aneesh.kumar@linux.ibm.com, anshuman.khandual@arm.com, apopple@nvidia.com, ardb@kernel.org, arnd@arndb.de, bp@alien8.de, cl@gentwo.org, dave.hansen@linux.intel.com, david@redhat.com, dennis@kernel.org, dev.jain@arm.com, dvyukov@google.com, glider@google.com, gwan-gyeong.mun@intel.com, hpa@zyccr.com, jane.chu@oracle.com, jgross@suse.de, jhubbard@nvidia.com, joao.m.martins@oracle.com, joro@8bytes.org, kas@kernel.org, kevin.brodsky@arm.com, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lorenzo.stoakes@oracle.com, luto@kernel.org, maobibo@loongson.cn, mhocko@suse.com, mingo@redhat.com, osalvador@suse.de, peterx@redhat.com, peterz@infradead.org, rppt@kernel.org, ryabinin.a.a@gmail.com, ryan.roberts@arm.com, stable@vger.kernel.org, surenb@google.com, tglx@linutronix.de, thuth@redhat.com, tj@kernel.org, urezki@gmail.com, vbabka@suse.cz, vincenzo.frascino@arm.com, x86@kernel.org, zhengqi.arch@bytedance.com References: <20250821093542.37844-1-harry.yoo@oracle.com> <20250821115731.137284-1-harry.yoo@oracle.com> <3976ef5d-a959-408a-b538-7feba1f0ab7a@intel.com> From: Dave Hansen Content-Language: en-US Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 8/21/25 18:11, Harry Yoo wrote: > On Thu, Aug 21, 2025 at 10:36:12AM -0700, Dave Hansen wrote: >> On 8/21/25 04:57, Harry Yoo wrote: >>> However, {pgd,p4d}_populate_kernel() is defined as a function regardless >>> of the number of page table levels, so the compiler may not optimize >>> them away. In this case, the following linker error occurs: > > Hi, thanks for taking a look, Dave! > > First of all, this is a fix-up patch of a mm-hotfixes patch series that > fixes a bug (I should have explained that in the changelog) [1]. > > [1] https://lore.kernel.org/linux-mm/20250818020206.4517-1-harry.yoo@oracle.com > > I think we can continue discussing it and perhaps do that as part of > a follow-up series, because the current patch series need to be backported > to -stable and your suggestion to improve existing code doesn't require > -stable backports. > > Does that sound fine? > >> This part of the changelog confused me. I think it's focusing on the >> wrong thing. >> >> The code that's triggering this is literally: >> >>> pgd_populate(&init_mm, pgd, >>> lm_alias(kasan_early_shadow_p4d)); >> >> It sure _looks_ like it's unconditionally referencing the >> 'kasan_early_shadow_p4d' symbol. I think it's wrong to hide that with >> macro magic and just assume that the macros won't reference it. >> >> If a symbol isn't being defined, it shouldn't be referenced in C code.:q > > A fair point, and that's what KASAN code has been doing for years. > >> The right way to do it is to have an #ifdef in a header that avoids >> compiling in the reference to the symbol. > > You mean defining some wrapper functions for p*d_populate_kernel() in > KASAN with different implementations based on ifdeffery? That would work. So would something like: #if CONFIG_PGTABLE_LEVELS >= 4 extern p4d_t kasan_early_shadow_p4d[MAX_PTRS_PER_P4D]; #else #define kasan_early_shadow_p4d NULL #endif > Just to clarify, what should be the exact ifdeffery to cover these cases? > #if CONFIG_PGTABLE_LEVELS == 4 and 5, or > #ifdef __PAGETABLE_P4D_FOLDED and __PAGETABLE_PUD_FOLDED ? > > I have no strong opinion on this, let's hear what KASAN folks think. I think CONFIG_PGTABLE_LEVELS works, but in the end I'm not picky about the specific #ifdefs that work.