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

[Feng Tang]

On Thu, Aug 30, 2018 at 03:25:42PM +0200, Thomas Gleixner wrote:
> On Thu, 30 Aug 2018, Feng Tang wrote:
> > On Thu, Aug 30, 2018 at 03:05:31PM +0200, Thomas Gleixner wrote:
> > > > 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.
> > > 
> > > And why do you want to use bootmem in the first place? If boot mem is going
> > > away, why are you not fixing the early console crap to NOT use bootmem at
> > > all?
> > 
> > The earlycon need use fixmap to map the mmio register, while fixmap will
> > need a new page for page table if the pmd/pte is not already their, as shown
> > in the commit log: 
> > 
> > 	  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
> > 
> > inside the spp_getpage():
> > 	
> > 	if (after_bootmem)
> > 		ptr = (void *) get_zeroed_page(GFP_ATOMIC);
> > 	else
> > 		ptr = alloc_bootmem_pages(PAGE_SIZE);
> 
> And where is the problem? We know that we need the fixmap during early boot
> anyway, so allocating the PTE page statically and setting it up early
> enough is not really rocket science. No allocator required.

Yes, if the bootmem itslef is going way, setting up the page table for
fixmap is a cleaner solution.

Thanks,
Feng