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 2F5BE3AE6EB; Wed, 13 May 2026 08:50:21 +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=1778662222; cv=none; b=fI4FWpw1tPEuuPwvGSMWQ+JFkg2MAlXZFOygctt8pEymOPtEtuxInUYFNEuL3EcygyrqVMoJeSPs1E4+siczhwymkd0A8KHG3RLYkWKcJccnDMaoRykAu0inp9l/PfnSuvxgsLqnQEXL2MB1hdcJQnUgpDt1ngkJrIlsXowBiB4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778662222; c=relaxed/simple; bh=N9P1iXvYrJqLgZmIOtPNBCWH68XujKCrRJM/d7OA1NE=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Uw9X+Hm5rV1e34obIJ31YtWSGEfwWb/ko6cAYVbhPAnNsgbxl5R3CagCzG94k6rMdTNZfv1c72iM3zx6MN+FsbGq8BZLUr6Ovm+glVqMc4dNBodGk2frMDzAjCfYuwpyYYnKtV8qREQkSI7LBLB1WbMLfEuLkM+hyUzZD1+C3lQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=P3wsuP8z; 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="P3wsuP8z" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 449B8C2BCB7; Wed, 13 May 2026 08:50:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778662221; bh=N9P1iXvYrJqLgZmIOtPNBCWH68XujKCrRJM/d7OA1NE=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=P3wsuP8z+CNKLupx+n1E2LTwB0iywnD4fV4upd41SWJq+wzKRFN+6l5o/Oie0iFXu QdpC0QzmJbU/K5OQ8IyHfr06kiJlM7LBMiHCJFHxnYf1+zAE/YVHre4MLSRYgTodzF 2UX6okTeCTJI9YtbbKEiw4HSykdkmljvjNwc9qLFqVNHKyLSJ8pz4UoB8aqnGx16Mp VHM75dVMdLqFiKF4Iy17YKPz7rWAnLkaR7h7HrJRw1/Fb3kroXQylFyEBZbcz6n1t4 Rrku0nAKE7qsstQILa3ifLy6O3io+f5iV74vmoN5D63J7MlkItbWbq+4JnF0SFHtfr lDHvYuOSl8IsA== Date: Wed, 13 May 2026 11:50:12 +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> 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: 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. > +#define EMPTY_ZERO_PAGE \ > + . = ALIGN(PAGE_SIZE); \ > + empty_zero_page = .; \ > + . += PAGE_SIZE; > +#else > +#define EMPTY_ZERO_PAGE > +#endif > +#endif > + > /* > * Read only Data > */ > @@ -479,6 +490,7 @@ > . = ALIGN((align)); \ > .rodata : AT(ADDR(.rodata) - LOAD_OFFSET) { \ > __start_rodata = .; \ > + EMPTY_ZERO_PAGE \ > *(.rodata) *(.rodata.*) *(.data.rel.ro*) \ > SCHED_DATA \ > -- Sincerely yours, Mike.