linux-kernel - Re: [PATCH 1/4] kexec: reuse crash kernel reserved memory for normal kexec

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Message-ID: <Yt6GPsWcG9iWHBUg@zx2c4.com>
Date:   Mon, 25 Jul 2022 14:02:06 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Albert Huang <huangjie.albert@...edance.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Michael Roth <michael.roth@....com>,
        Nathan Chancellor <nathan@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Joerg Roedel <jroedel@...e.de>,
        Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org, kexec@...ts.infradead.org,
        linux-kbuild@...r.kernel.org
Subject: Re: [PATCH 1/4] kexec: reuse crash kernel reserved memory for normal
 kexec

Hi Albert,

On Mon, Jul 25, 2022 at 04:38:53PM +0800, Albert Huang wrote:
> The kexec userspace tool also needs to add parameter options(-r) that
> support the use of reserved memory (see another patch for kexec)
>
> [...]
>
> -	if (kexec_on_panic) {
> +	if (kexec_on_panic && kexec_on_reserved) {

Two small questions related to this:

- Why does kexec-tools need an option, or more specifically, why does
  userspace need to communicate about this at all? Can't the kernel just
  automatically use the available reserved memory in the case that's not
  already being used by the panic handler kernel? I'm curious about
  whether there are caveats that would make this occasionally
  undesirable, hence suggesting an option.

- I don't totally understand how this works, so I might be a bit off
  here, but is there any chance that this could be made to co-exist with
  kexec_on_panic? Can a larger region just be reserved, specifically for
  this, rather than piggy backing on the panic handler region?

> +static struct page *kimage_alloc_reserverd_control_pages(struct kimage *image,
> +	case KEXEC_TYPE_RESERVED_MEM:
> +		pages = kimage_alloc_reserverd_control_pages(image, order);

Nit:
  reserverd -> reserved

Jason