linux-kernel - Re: [PATCH] x86, mm: Reserver some memory for bootmem allocator for NO

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Date:   Thu, 30 Aug 2018 20:59:09 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        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

On Thu, Aug 30, 2018 at 02:49:15PM +0200, Michal Hocko wrote:
> On Thu 30-08-18 13:54:19, Thomas Gleixner wrote:
> > On Thu, 30 Aug 2018, Michal Hocko wrote:
> > > On Thu 30-08-18 12:44:02, Peter Zijlstra wrote:
> > > > On Thu, Aug 30, 2018 at 05:03:19PM +0800, Feng Tang wrote:
> > > > > 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.
> > > 
> > > Well, there is endeavor to remove bootmem allocator altogether. So
> > > making earlycon depend on NO_BOOTMEM=n doesn't sound like a good fit to
> > 
> > If we want to remove bootmem, then reintroducing it with a static bootmem
> > section doesn't make any sense at all.
> 
> I have actually checked the code now and see what you mean. I thought it
> would be a single allocation that is needed but that is not the case so
> the static buffer is not going to fly.

Exactly! I tried several ways for the static allocation, like in data, in bss
section, but they all failed, as in the very start of setup_arch():

	memblock_reserve(__pa_symbol(_text),
		(unsigned long)__bss_stop - (unsigned long)_text);

Those [_text, __bss_stop] is not able to be used by alloc_bootmem(). And I
only got this patch, and really appreciate any good suggestions.

Thanks,
Feng