linux-kernel - Re: [PATCH V3 07/17] riscv: compat: Re-implement TASK_SIZE for COMPAT

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Date:   Thu, 20 Jan 2022 15:57:27 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Guo Ren <guoren@...nel.org>
Cc:     David Laight <David.Laight@...lab.com>,
        Arnd Bergmann <arnd@...db.de>,
        Palmer Dabbelt <palmer@...belt.com>,
        Anup Patel <anup@...infault.org>,
        gregkh <gregkh@...uxfoundation.org>,
        liush <liush@...winnertech.com>, Wei Fu <wefu@...hat.com>,
        Drew Fustini <drew@...gleboard.org>,
        Wang Junqiang <wangjunqiang@...as.ac.cn>,
        Christoph Hellwig <hch@....de>,
        Christoph Hellwig <hch@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        "linux-csky@...r.kernel.org" <linux-csky@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        sparclinux <sparclinux@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        "inux-parisc@...r.kernel.org" <inux-parisc@...r.kernel.org>,
        "open list:BROADCOM NVRAM DRIVER" <linux-mips@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Guo Ren <guoren@...ux.alibaba.com>
Subject: Re: [PATCH V3 07/17] riscv: compat: Re-implement TASK_SIZE for COMPAT_32BIT

On Thu, Jan 20, 2022 at 2:27 PM Guo Ren <guoren@...nel.org> wrote:
> On Thu, Jan 20, 2022 at 8:53 PM David Laight <David.Laight@...lab.com> wrote:
> > I think the x86-64 32bit compat code even puts the boundary at 4GB.
> Yes, we could give rv32 compat for 4GB with some effort. But it's unnecessary.
>
> There are no history issues for rv32, we use compat mode to reduce
> memory footprint. eg: only 64MB memory available.
>
> At end compat for 4GB is another topic, let's give the initial compat
> for 2GB support to riscv.

I think it's fine either way. Having the entire 4GB space available is nice
when you are trying to build 32-bit software natively rather then using a
cross-compiler, as you can just do it on a larger machine that supports both.

One example of software that runs into virtual memory size limitations is
the gnu linker when building large applications, but it's unlikely that you'll
actually need to run applications that run into this, while also needing to
build them natively.

Using the same limit as on native 32-bit machines can help with compatibility
of certain software, but again this is rarely a problem and I have not seen any
reports of issues with the 4GB TASK_SIZE_32 on arm64. On x86, there
is an option to use the native 3GB TASK_SIZE for compat tasks. This was
introduced to work around buggy applications a long time ago, but is
probably not used any more in practice.

       Arnd