From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 2BF661862; Wed, 13 May 2026 10:29:00 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778668141; cv=none; b=GO4+BKuIyZK1ug+cLae7J6GH3Ipyuu+HEBQEAFKJTyHC/z4OyRbwP5/tVpjFekvHmt0Gcw/X5a4tivzkEXKXjwxd7sOhsGOCgv154+hBnKEuLqU1JtUAkvwwe9M+tUy3kD3vSTDOeeHdKw3Gxzzve2qfpVcJcaMD9nniHBl7Qbo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778668141; c=relaxed/simple; bh=SMSduI5AnAM9kqchaE8jYdlaOT2npCWPyM5+zGPU3o0=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=FYDKM1ik/3O9B5HqW/1wmzAKyEiiKg6Jp6SMjNPd1OSzHBFNyBIAXFff9947oJf3aRJHqhs3O208Ds/1HzW/9Nnpyqym0OepZs0e2qviETBITIxt7XY8ksdYpvx04MmEAFjAKJ7mqOmGz2RgnhcZZdytVVPm7z0JoSu2YZcLvXI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=oqTXi24H; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="oqTXi24H" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6F393C2BCB7; Wed, 13 May 2026 10:28:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778668140; bh=SMSduI5AnAM9kqchaE8jYdlaOT2npCWPyM5+zGPU3o0=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=oqTXi24HsgBOAFuKC6Vi/Jw3ToZ3ix8HWddmTwJTDN0Un1xAN5BkYQzXEShkWawTo GbwdlpXAgWvDqvcNS4fj4SamaL21POucJDoSE7gUJ+y6jgqAQeAzM1iOpDyFP3dDYj RbhQq9r3uXj5eja2m/YDTvPvES31K7BtxsHNPpQukkzx1jXDmNwQs88p5AhLbZkYxo WYY1U+LSv7W2Mu8THlmQACiifeBWhl6VW8dk+ILwXNmNO/9dTqLE3RnuJo2ptdVwoS hfHoLk3RKGkwl/5TkgZ09NqeNYyFzaQwl3SN0CdD5rL7XU7NeUhJHKrDEBwv+tCA7/ ezv48qxYD1c9Q== Date: Wed, 13 May 2026 13:28:51 +0300 From: Mike Rapoport To: Ard Biesheuvel Cc: Jann Horn , Ard Biesheuvel , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Will Deacon , Catalin Marinas , Mark Rutland , Ryan Roberts , Anshuman Khandual , Liz Prucka , Seth Jenkins , Kees Cook , David Hildenbrand , Andrew Morton , linux-mm@kvack.org, linux-hardening@vger.kernel.org Subject: Re: [PATCH v4 02/15] mm: Make empty_zero_page __ro_after_init Message-ID: References: <20260427153416.2103979-17-ardb+git@google.com> <20260427153416.2103979-19-ardb+git@google.com> <31252c1d-a98d-4635-ab61-ce5b649e256f@app.fastmail.com> <49d30ffe-b7e1-4b05-89e0-c3fe01348bb6@app.fastmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <49d30ffe-b7e1-4b05-89e0-c3fe01348bb6@app.fastmail.com> On Wed, May 13, 2026 at 10:53:08AM +0200, Ard Biesheuvel wrote: > > On Wed, 13 May 2026, at 10:50, Mike Rapoport wrote: > > On Tue, May 12, 2026 at 02:56:16PM +0200, Ard Biesheuvel wrote: > >> On Mon, 11 May 2026, at 16:40, Jann Horn wrote: > >> > On Mon, May 11, 2026 at 10:59 AM Ard Biesheuvel wrote: > >> >> I think we should simply do something along the lines of the below, > >> >> considering that the size of a data object tends to correlate with > >> >> its minimum alignment. > >> >> > >> >> I do find it rather puzzling that the compiler emits empty_zero_page > >> >> *after* zero_page_pfn - ideally, we'd combine the below with > >> >> -fdata-sections so that the linker sees all individual objects, but > >> >> I suspect that would create some problems elsewhere. > >> >> > >> >> > >> >> --- a/include/asm-generic/vmlinux.lds.h > >> >> +++ b/include/asm-generic/vmlinux.lds.h > >> >> @@ -452,7 +452,7 @@ > >> >> #define RO_AFTER_INIT_DATA \ > >> >> . = ALIGN(8); \ > >> >> __start_ro_after_init = .; \ > >> >> - *(.data..ro_after_init) \ > >> >> + *(SORT_BY_ALIGNMENT(.data..ro_after_init)) \ > >> > > >> > Oh, neat, I didn't realize that's possible. That seems like a nicer > >> > approach... > >> > >> Neat but rather ineffective, unfortunately. (I don't see a size > >> difference with the arm64 defconfig kernel) > >> > >> Given that empty_zero_page only ever gets its address taken, we > >> might just move it into the linker script if that requires tweaking > >> anyway. We can just place it at the start of .rodata, which is > >> already page aligned on most architectures (and will become page > >> aligned unless EMPTY_ZERO_PAGE is #define'd by the arch linker > >> script to something else) > >> > >> > >> --- a/include/asm-generic/vmlinux.lds.h > >> +++ b/include/asm-generic/vmlinux.lds.h > >> @@ -472,6 +472,17 @@ > >> #endif > >> #endif > >> > >> +#ifndef EMPTY_ZERO_PAGE > >> +#ifndef __HAVE_COLOR_ZERO_PAGE > > > > I don't think we want let architectures that don't use colored zero pages > > redefine it. > > If it will be really required we can add the ability to redefine > > EMPTY_ZERO_PAGE later. > > > > I was actually intending to add use this for arm64 in the next patch. It > already has a reserved_pg_dir in .rodata which is page-sized (i.e., up > to 64k in size) and guaranteed to remain all zeroes, so empty_zero_page > could actually be an alias for that. This is what I had in a previous > revision, before you turned the empty_zero_page definition into common > code: Works for me if from arm64 perspective that's ok :) > https://lore.kernel.org/all/20260320145934.2349881-16-ardb+git@google.com/ -- Sincerely yours, Mike.