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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> 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> X-Rspam-User: X-Rspamd-Queue-Id: DB4D040008 X-Rspamd-Server: rspam06 X-Stat-Signature: ka7qy5ycfsi36c774jxb393564fucpoz X-HE-Tag: 1778668141-506188 X-HE-Meta: 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 iB7hW63c L6PWUo040TgYUYDlvA8IeWY+abfH/RJ3O0roUR60fj3Cn5IwR+iUWZMBdjX81KaZgZLd+QEmURQ6uX+w8KP53VDzaP64rHBPRFUNRh0C7mkVUAhGIc+3/HkvdX2+eGgPTyvEb9RZn4XmPQyDHH8QLtHPWsJ1h8RaXVhhryJLthaduhMj/XtX72rUENjx4NSyTFEZV+vHVdM076uWymCf7NFLasMie8dk26vzBWIsQhnpDMnMPyHdqUbHFrpEIYsQ7mbAfk44SP0UAXIwywNME5EuhDvKa6b8ohkx1qFTFVs2k8fESJ5dNR/pYa3zVuNlS3S1b6Cv7U61eUrN00P/tIQeNVe4hkd8/4l4tavB1ITsx2W3h8mXYCe/BiJJMtefzKSsoZDxUeUDW8+wnsLDrHZl4jKDRW15OeRHKeqE8V4HyR1fsnzxmY2mym9lfs7mWIKJq Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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.