From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 464927081A for ; Thu, 17 Sep 2026 18:28:18 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789669699; cv=none; b=s1VKIEd2xb+qP5IILvYcW2kpoVM0h1RorV4Iyi6eUW6W72o2kqGPQE47sivTV0X8M2aRZeJtJrFA8h7SU3mvmV47fYLeLdMw8TqzYoLkOQMTLoFhPC6FKTDnkfIOY+hku+EepAvGtaijWyVAnj22D3aqzlTlMrtKv8JW17xsuJI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789669699; c=relaxed/simple; bh=PlN1J4bq5Zd5rPbVN3If9wdWVIq+fnSAryeGXeIQVBc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=QZkjt6DuQKKCwY7wVxIowZjQCZDKwgjMeXShejw3IL1MtLTGLK/KEXZ2T+k4zDaGxXKkVxZQwx0EwDavCbDz697Bbfyk4Mm9vM52Qod9hRhC/ERfH+Vwl1MZOLAbQ2x8EIhW0NbK5G8rR8ZNMnrpZv/QJuFbRXml32N+2qFnahU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=hx4f6/Jv; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="hx4f6/Jv" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C4D881F000FF; Thu, 17 Sep 2026 18:28:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789669697; bh=b5zVLhEkKOWEwakOfls77zzn3VcbclTffxYn4L85i8c=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=hx4f6/Jv3cwrMptmmP/azM4P7NNl4TgzLMYGPYxUvgj5fA03oSL4gHfl55lsCvHTT pn5EvppKp1xmJVGJoCg35YpgKo//7RnjqDcC6S8a+5bv13BmFQXoavodtPLD14ATOC rmLLzfgKdbwPSczeH8sX7PRwlBQtwwRWVY2CYFlhqSagmfzeq2QSmTyLzn5Sn1v1ez GsYSrSrknJk0CULzb4njUW5QsatEUpC4El2M6cPUEGGYdRiyicyK7Mntpp8rww/Avk Qil1cbyfX1lJNXxc5p1cZu4mSWyl9wU4Cbmp/v2R2k2C3fUf7DbdZRhNL48WnmlEFp ySmtFZ+wMOr1w== Message-ID: Date: Thu, 17 Sep 2026 20:28:10 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH V2 06/14] arm64/mm: Enable fixmap with 5 level page table To: Anshuman Khandual Cc: linux-arm-kernel@lists.infradead.org, Catalin Marinas , Will Deacon , Ryan Roberts , Mark Rutland , Lorenzo Stoakes , Andrew Morton , Mike Rapoport , Linu Cherian , linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20260907035030.2839978-1-anshuman.khandual@arm.com> <20260907035030.2839978-7-anshuman.khandual@arm.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 9/17/26 08:18, Anshuman Khandual wrote: > On Wed, Sep 16, 2026 at 04:30:05PM +0200, David Hildenbrand (Arm) wrote: >> On 9/7/26 05:50, Anshuman Khandual wrote: >>> FEAT_D128 halves PTRS_PER_PXX thus shrinking the VA range coverage >>> for each page table level. Hence in order to preserve all existing >>> VA range configurations, some geometry now need to become 5-level. >>> >> >> Hmm, wasn't this 5-levels before, but other code allocated+initialized the p4d >> table for us? > > But fixmap was never 5 level earlier. I mean the page table hierarchy. It's confusing. > >> >> I mean, I see how we are filling in another level, but I don't see how we would >> reconfigure something to use 5 page table levels? > > IIUC existing configurations which never required 5 levels would not need > this reconfiguration even now. We only map a handful of MB into a single leaf page table. The only reason why we have to install more page tables into upper levels is because they were not instantiated by other code. > >> >>> Since fixmap is used to build and manipulate page tables early on >>> during boot the mapping must also gain that additional level which >>> was not required earlier. >> >> I am no expert ... but I am confused :) >> >> We do have a loop only in early_fixmap_init_pmd(). So we cannot possibly span >> more than a single ... PTE table? >> >>> >>> Enable fixmap with 5 level page table when required. This creates table >>> entries at the PGD level. Add a fallback stub for pgd_page_paddr() when >>> (PGTBALE_LEVELS <= 4) which helps in intercepting any unintended usage. >> >> >> Again, I am very confused. Is it really just "As the page table layout changes, >> we might no longer have a p4d table pre-populated for the range our fixmap falls >> into. So if there is no p4d table yet, populate a static one just like we do for >> the other levels." > > Right. Best to rewrite the patch description along these lines. -- Cheers, David