linux-kernel - Re: [PATCH 6.4 00/28] 6.4.1-rc1 review

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Message-ID: <CAHk-=whh_aUHYF6LCV36K9NYHR4ofEZ0gwcg0RY5hj=B7AT4YQ@mail.gmail.com>
Date:   Fri, 30 Jun 2023 18:24:49 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Guenter Roeck <linux@...ck-us.net>,
        Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>
Cc:     Naresh Kamboju <naresh.kamboju@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, patches@...ts.linux.dev,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        shuah@...nel.org, patches@...nelci.org,
        lkft-triage@...ts.linaro.org, pavel@...x.de, jonathanh@...dia.com,
        f.fainelli@...il.com, sudipm.mukherjee@...il.com,
        srw@...dewatkins.net, rwarsow@....de, conor@...nel.org,
        linux-parisc <linux-parisc@...r.kernel.org>,
        sparclinux@...r.kernel.org,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Helge Deller <deller@....de>,
        Jason Wang <wangborong@...rlc.com>
Subject: Re: [PATCH 6.4 00/28] 6.4.1-rc1 review

On Fri, 30 Jun 2023 at 15:51, Guenter Roeck <linux@...ck-us.net> wrote:
>
> There is one more, unfortunately.
>
> Building xtensa:de212:kc705-nommu:nommu_kc705_defconfig ... failed

Heh. I didn't even realize that anybody would ever do
lock_mm_and_find_vma() code on a nommu platform.

With nommu, handle_mm_fault() will just BUG(), so it's kind of
pointless to do any of this at all, and I didn't expect anybody to
have this page faulting path that just causes that BUG() for any
faults.

But it turns out xtensa has a notion of protection faults even for
NOMMU configs:

    config PFAULT
        bool "Handle protection faults" if EXPERT && !MMU
        default y
        help
          Handle protection faults. MMU configurations must enable it.
          noMMU configurations may disable it if used memory map never
          generates protection faults or faults are always fatal.

          If unsure, say Y.

which is why it violated my expectations so badly.

I'm not sure if that protection fault handling really ever gets quite
this far (it certainly should *not* make it to the BUG() in
handle_mm_fault()), but I think the attached patch is likely the right
thing to do.

Can you check if it fixes that xtensa case? It looks
ObviouslyCorrect(tm) to me, but considering that I clearly missed this
case existing AT ALL, it might be best to double-check.

               Linus

View attachment "patch.diff" of type "text/x-patch" (1876 bytes)