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

[Mike Rapoport]

On Wed, Apr 14, 2021 at 05:52:57PM +0200, David Hildenbrand wrote:
> On 14.04.21 17:27, Ard Biesheuvel wrote:
> > On Wed, 14 Apr 2021 at 17:14, David Hildenbrand <david@xxxxxxxxxx> wrote:
> > > 
> > > On 07.04.21 19:26, Mike Rapoport wrote:
> > > > From: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> > > > 
> > > > 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.

Currently we have memmap entries with struct page set to defaults for the
NOMAP memory. AFAIU hiding them in pfn_valid()/pfn_valid_within() was a
solution to failures in pfn walkers that presumed that for a pfn_valid()
there will be a struct page that really reflects the state of that page.

> 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().

Not sure we really need a new pagetype here, PG_Reserved seems to be quite
enough to say "don't touch this".  I generally agree that we could make
PG_Reserved a PageType and then have several sub-types for reserved memory.
This definitely will add clarity but I'm not sure that this justifies
amount of churn and effort required to audit uses of PageResrved().
 
> 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. 

Stop pushing memblock out of kernel! ;-)

Now, seriously, we can minimize memblock involvement in run-time and this
series in yet another step in that direction.

-- 
Sincerely yours,
Mike.