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Date:   Thu, 30 Aug 2018 20:43:42 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...e.hu>,
        "H . Peter Anvin" <hpa@...ux.intel.com>,
        Yinghai Lu <yinghai@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] x86, mm: Reserver some memory for bootmem allocator for
 NO_BOOTMEM

Hi Peter,

On Thu, Aug 30, 2018 at 12:44:02PM +0200, Peter Zijlstra wrote:
> On Thu, Aug 30, 2018 at 05:03:19PM +0800, Feng Tang wrote:
> > We hit a kernel panic when enabling earlycon for a platform, the
> > call trace is:
> > 
> > 	  panic+0xd2/0x220
> > 	  __alloc_bootmem+0x31/0x34
> > 	  spp_getpage+0x60/0x8a
> > 	  fill_pte+0x71/0x130
> > 	  __set_pte_vaddr+0x1d/0x50
> > 	  set_pte_vaddr+0x3c/0x60
> > 	  __native_set_fixmap+0x23/0x30
> > 	  native_set_fixmap+0x30/0x40
> > 	  setup_earlycon+0x1e0/0x32f
> > 	  param_setup_earlycon+0x13/0x22
> > 	  do_early_param+0x5b/0x90
> > 	  parse_args+0x1f7/0x300
> > 	  parse_early_options+0x24/0x28
> > 	  parse_early_param+0x65/0x73
> > 	  setup_arch+0x31e/0x9f1
> > 	  start_kernel+0x58/0x44e
> > 
> > The root cause is that when CONFIG_NO_BOOTMEM=y,  before
> > e820__memblock_setup() is called there is no memory for bootmem
> > to allocate,
> 
> Which you bloody well asked for by using NO_BOOTMEM=y.
> 
> Going down this route; adding hacks for every little thing that does
> want bootmem, completely defeats the purpose.
> 
> If anything, make the earlycon thing depend on NO_BOOTMEM=n. That also
> solves your problem. No earlycon, no panic.

In current x86, the NO_BOOTMEM is "y" by default, and almost all the
servers/desktops I could access have CONFIG_BOOTMEM=y, while earlycon
is important for enabling new platforms, as well as kernel debugging.
IMHO, it's better to keep earlycon capability. Also any alloc_bootmem()
call in that time window will trigger panic.

Thanks,
Feng

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