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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> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 0FF401A0004 X-Stat-Signature: qjiz81jdypea11z8gzx4otri16hbe1r1 X-HE-Tag: 1778662222-785134 X-HE-Meta: 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 As0pSMM4 xGZ8Ed7l4miaZS0V09bXBg/cdYYDGDlsGhJf2M0X6VrAiJCwZw8IT+y0AtjUBSD6oIusa1gGeEhB4mox3eWZ0WNxZaTisca/QmN3ATJWY+714k80QxatZo5Sf764/3DXtxypwIjBaUNpIbzs1ijjjJx6gOqDzr6kjPxaSFT3AYM+NZnLITieapxfmLlWUcBZG0bb3JiftasgcQfvNDeLE90STD1wkVcRot9lJH3/+4AZAqS1duXhM/Ft1ttvDxS33QfUaYsVdgW3fv1twu4qXiQVd9AEDGNKWjyl3e+GMOQwVCrjp9tq8fhz7vkefO73q8X7yzIYKOurthHJ/SlvSt7YY9wBPIoxCMGgwnUy5KkJcHnitjxxABZBqlGiVijcoHicFe4cgfeyUYJHj78zs1Y1ULACqTRvkam2lav0JLSJH770= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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.