linux-kernel - Re: [PATCH V4 8/8] riscv: Add config of thread stack size

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Date:   Sun, 11 Sep 2022 20:39:35 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     guoren <guoren@...nel.org>
Cc:     "Palmer Dabbelt" <palmer@...osinc.com>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Peter Zijlstra" <peterz@...radead.org>,
        "Andy Lutomirski" <luto@...nel.org>,
        "Conor.Dooley" <conor.dooley@...rochip.com>,
        Heiko Stübner <heiko@...ech.de>,
        "Jisheng Zhang" <jszhang@...nel.org>, lazyparser@...il.com,
        falcon@...ylab.org, "Huacai Chen" <chenhuacai@...nel.org>,
        "Anup Patel" <apatel@...tanamicro.com>,
        "Atish Patra" <atishp@...shpatra.org>,
        "Palmer Dabbelt" <palmer@...belt.com>,
        "Paul Walmsley" <paul.walmsley@...ive.com>,
        "Sebastian Andrzej Siewior" <bigeasy@...utronix.de>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org,
        "Guo Ren" <guoren@...ux.alibaba.com>,
        "Andreas Schwab" <schwab@...e.de>
Subject: Re: [PATCH V4 8/8] riscv: Add config of thread stack size



On Sun, Sep 11, 2022, at 1:35 AM, Guo Ren wrote:
> On Sun, Sep 11, 2022 at 12:07 AM Arnd Bergmann <arnd@...db.de> wrote:
>>
>> On Sat, Sep 10, 2022, at 2:52 PM, Guo Ren wrote:
>> > On Thu, Sep 8, 2022 at 3:37 PM Arnd Bergmann <arnd@...db.de> wrote:
>> >> On Thu, Sep 8, 2022, at 4:25 AM, guoren@...nel.org wrote:
>> >> > From: Guo Ren <guoren@...ux.alibaba.com>
>> >> - When VMAP_STACK is set, make it possible to select non-power-of-two
>> >>   stack sizes. Most importantly, 12KB should be a really interesting
>> >>   choice as 8KB is probably still not enough for many 64-bit workloads,
>> >>   but 16KB is often more than what you need. You probably don't
>> >>   want to allow 64BIT/8KB without VMAP_STACK anyway since that just
>> >>   makes it really hard to debug, so hiding the option when VMAP_STACK
>> >>   is disabled may also be a good idea.
>> > I don't want this config to depend on VMAP_STACK. Some D1 chips would
>> > run with an 8K stack size and !VMAP_STACK.
>>
>> That sounds like a really bad idea, why would you want to risk
>> using such a small stack without CONFIG_VMAP_STACK?
>>
>> Are you worried about increased memory usage or something else?
> The requirement is from [1], and I think disabling CONFIG_VMAP_STACK
> would be the last step after serious testing.

I still don't see why you need to turn off VMAP_STACK at all
if it works. The only downside I can see with using VMAP_STACK
on a 64-bit system is that it may expose bugs with device
drivers that do DMA to stack data. Once you have tested the
system successfully, you can also assume that you have no such
drivers.

     Arnd