Re: [PATCH] arm64: mm: Fix memmap to be initialized for the entire section

[Will Deacon <will.deacon@arm.com>]

On Fri, Oct 28, 2016 at 11:19:05AM +0200, Robert Richter wrote:
> On 27.10.16 17:01:36, Will Deacon wrote:
> > It feels to me like NOMAP memory is a new type
> > of memory where there *is* a struct page, but it shouldn't be used for
> > anything.
> 
> IMO, a NOMAP page should just be handled like a reserved page except
> that the page is marked reserved. See free_low_memory_core_early().
> Thus, NOMAP pages are not in the free pages list or set to reserved.
> It is simply not available for mapping at all. Isn't that exactly what
> it should be?
> 
> I also did not yet understand the benefit of the differentiation
> between NOMAP and reserved and the original motivation for its
> implementation. I looked through the mail threads but could not find
> any hint. The only difference I see now is that it is not listed as a
> reserved page, but as long as it is not freed it should behave the
> same. I remember the case to handle memory different (coherency,
> etc.), but are not sure here. Ard, could you explain this?
> 
> > I don't think pfn_valid can describe that, given the way it's
> > currently used, and flipping the logic is just likely to move the problem
> > elsewhere.
> > 
> > What options do we have for fixing this in the NUMA code?
> 
> Out of my mind:
> 
> 1) Treat NOMAP pages same as reserved pages (my patch).

Just to reiterate here, but your patch as it stands will break other parts
of the kernel. For example, acpi_os_ioremap relies on being able to ioremap
these regions afaict.

I think any solution involving pfn_valid is just going to move the crash
around.

Will