Re: [PATCHv2 5/7] x86/mm: Reserve unaccepted memory bitmap

[Mike Rapoport]

On Wed, Jan 12, 2022 at 11:53:42AM -0800, Dave Hansen wrote:
> On 1/12/22 11:43 AM, Kirill A. Shutemov wrote:
> > On Tue, Jan 11, 2022 at 11:10:40AM -0800, Dave Hansen wrote:
> >> On 1/11/22 03:33, Kirill A. Shutemov wrote:
> >>
> >>> +	/* Mark unaccepted memory bitmap reserved */
> >>> +	if (boot_params.unaccepted_memory) {
> >>> +		unsigned long size;
> >>> +
> >>> +		/* One bit per 2MB */
> >>> +		size = DIV_ROUND_UP(e820__end_of_ram_pfn() * PAGE_SIZE,
> >>> +				    PMD_SIZE * BITS_PER_BYTE);
> >>> +		memblock_reserve(boot_params.unaccepted_memory, size);
> >>> +	}
> >>
> >> Is it OK that the size of the bitmap is inferred from
> >> e820__end_of_ram_pfn()?  Is this OK in the presence of mem= and other things
> >> that muck with the e820?
> > 
> > Good question. I think we are fine. If kernel is not able to allocate
> > memory from a part of physical address space we don't need the bitmap for
> > it either.
> 
> That's a good point.  If the e820 range does a one-way shrink it's
> probably fine.  The only problem would be if the bitmap had space for
> for stuff past e820__end_of_ram_pfn() *and* it later needed to be accepted.

It's unlikely, but e820 can grow because of EFI and because of memmap=.
To be completely on the safe side, the unaccepted bitmap should be reserved
after parse_early_param() and efi_memblock_x86_reserve_range().

Since we anyway do not have memblock allocations before
e820__memblock_setup(), the simplest thing would be to put the reservation
first thing in e820__memblock_setup().

-- 
Sincerely yours,
Mike.