linux-kernel - Re: [PATCH v2 07/12] arm64, execmem: extend execmem

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Date:   Sat, 17 Jun 2023 16:37:29 -0400
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Song Liu <song@...nel.org>
Cc:     Mike Rapoport <rppt@...nel.org>, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        "David S. Miller" <davem@...emloft.net>,
        Dinh Nguyen <dinguyen@...nel.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Helge Deller <deller@....de>,
        Huacai Chen <chenhuacai@...nel.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nadav Amit <nadav.amit@...il.com>,
        "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Puranjay Mohan <puranjay12@...il.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Russell King <linux@...linux.org.uk>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, bpf@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-mips@...r.kernel.org,
        linux-mm@...ck.org, linux-modules@...r.kernel.org,
        linux-parisc@...r.kernel.org, linux-riscv@...ts.infradead.org,
        linux-s390@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, loongarch@...ts.linux.dev,
        netdev@...r.kernel.org, sparclinux@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v2 07/12] arm64, execmem: extend execmem_params for
 generated code definitions

On Sat, Jun 17, 2023 at 09:38:17AM -0700, Song Liu wrote:
> On Sat, Jun 17, 2023 at 8:37 AM Kent Overstreet
> <kent.overstreet@...ux.dev> wrote:
> >
> > On Sat, Jun 17, 2023 at 09:57:59AM +0300, Mike Rapoport wrote:
> > > > This is growing fast. :) We have 3 now: text, data, jit. And it will be
> > > > 5 when we split data into rw data, ro data, ro after init data. I wonder
> > > > whether we should still do some type enum here. But we can revisit
> > > > this topic later.
> > >
> > > I don't think we'd need 5. Four at most :)
> > >
> > > I don't know yet what would be the best way to differentiate RW and RO
> > > data, but ro_after_init surely won't need a new type. It either will be
> > > allocated as RW and then the caller will have to set it RO after
> > > initialization is done, or it will be allocated as RO and the caller will
> > > have to do something like text_poke to update it.
> >
> > Perhaps ro_after_init could use the same allocation interface and share
> > pages with ro pages - if we just added a refcount for "this page
> > currently needs to be rw, module is still loading?"
> 
> If we don't relax rules with read only, we will have to separate rw, ro,
> and ro_after_init. But we can still have page sharing:
> 
> Two modules can put rw data on the same page.
> With text poke (ro data poke to be accurate), two modules can put
> ro data on the same page.
> 
> > text_poke() approach wouldn't be workable, you'd have to audit and fix
> > all module init code in the entire kernel.
> 
> Agreed. For this reason, each module has to have its own page(s) for
> ro_after_init data.

Relaxing page permissions to allow for page sharing could also be a
config option. For archs with 64k pages it seems worthwhile.