Re: [PATCH] x86, mm: Reserver some memory for bootmem allocator for NO_BOOTMEM

[Michal Hocko <mhocko@kernel.org>]

On Thu 30-08-18 14:09:26, Peter Zijlstra wrote:
> On Thu, Aug 30, 2018 at 01:12:07PM +0200, 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:
> > > > 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.
> > 
> > Well, there is endeavor to remove bootmem allocator altogether. So
> 
> wasn't aware of that. why?

Because it is yet another allocator that we have to maintain and that is
not really needed.
 
> > making earlycon depend on NO_BOOTMEM=n doesn't sound like a good fit to
> > me. I am not familiar with this code path but why cannot we postpone the
> > allocation to later or use a statically allocated storage?
> 
> because 'early'... I suppose. Youu really want the early con up and
> running asap.

Well, the memblock allocator is initialized quite early as well... If
that is not early enough then we have to play tricks.
-- 
Michal Hocko
SUSE Labs