Re: [RFC/RFT PATCH 1/3] memblock: update initialization of reserved pages

[David Hildenbrand <david@redhat.com>]

On 14.04.21 17:27, Ard Biesheuvel wrote:
> On Wed, 14 Apr 2021 at 17:14, David Hildenbrand <david@redhat.com> wrote:
>>
>> On 07.04.21 19:26, Mike Rapoport wrote:
>>> From: Mike Rapoport <rppt@linux.ibm.com>
>>>
>>> The struct pages representing a reserved memory region are initialized
>>> using reserve_bootmem_range() function. This function is called for each
>>> reserved region just before the memory is freed from memblock to the buddy
>>> page allocator.
>>>
>>> The struct pages for MEMBLOCK_NOMAP regions are kept with the default
>>> values set by the memory map initialization which makes it necessary to
>>> have a special treatment for such pages in pfn_valid() and
>>> pfn_valid_within().
>>
>> I assume these pages are never given to the buddy, because we don't have
>> a direct mapping. So to the kernel, it's essentially just like a memory
>> hole with benefits.
>>
>> I can spot that we want to export such memory like any special memory
>> thingy/hole in /proc/iomem -- "reserved", which makes sense.
>>
>> I would assume that MEMBLOCK_NOMAP is a special type of *reserved*
>> memory. IOW, that for_each_reserved_mem_range() should already succeed
>> on these as well -- we should mark anything that is MEMBLOCK_NOMAP
>> implicitly as reserved. Or are there valid reasons not to do so? What
>> can anyone do with that memory?
>>
>> I assume they are pretty much useless for the kernel, right? Like other
>> reserved memory ranges.
>>
> 
> On ARM, we need to know whether any physical regions that do not
> contain system memory contain something with device semantics or not.
> One of the examples is ACPI tables: these are in reserved memory, and
> so they are not covered by the linear region. However, when the ACPI
> core ioremap()s an arbitrary memory region, we don't know whether it
> is mapping a memory region or a device region unless we keep track of
> this in some way. (Device mappings require device attributes, but
> firmware tables require memory attributes, as they might be accessed
> using misaligned reads)

Using generically sounding NOMAP ("don't create direct mapping") to 
identify device regions feels like a hack. I know, it was introduced 
just for that purpose.

Looking at memblock_mark_nomap(), we consider "device regions"

1) ACPI tables

2) VIDEO_TYPE_EFI memory

3) some device-tree regions in of/fdt.c


IIUC, right now we end up creating a memmap for this NOMAP memory, but 
hide it away in pfn_valid(). This patch set at least fixes that.

Assuming these pages are never mapped to user space via the struct page 
(which better be the case), we could further use a new pagetype to mark 
these pages in a special way, such that we can identify them directly 
via pfn_to_page().

Then, we could mostly avoid having to query memblock at runtime to 
figure out that this is special memory. This would obviously be an 
extension to this series. Just a thought.

-- 
Thanks,

David / dhildenb