Re: [RFC 1/2] arm64/mm: Fix pfn_valid() for ZONE_DEVICE based memory

Mike Rapoport
2021-01-25T07:33:59Z

On Mon, Jan 25, 2021 at 11:52:32AM +0530, Anshuman Khandual wrote:
>
> On 12/22/20 12:42 PM, Anshuman Khandual wrote:
> > pfn_valid() asserts that there is a memblock entry for a given pfn without
> > MEMBLOCK_NOMAP flag being set. The problem with ZONE_DEVICE based memory is
> > that they do not have memblock entries. Hence memblock_is_map_memory() will
> > invariably fail via memblock_search() for a ZONE_DEVICE based address. This
> > eventually fails pfn_valid() which is wrong. memblock_is_map_memory() needs
> > to be skipped for such memory ranges. As ZONE_DEVICE memory gets hotplugged
> > into the system via memremap_pages() called from a driver, their respective
> > memory sections will not have SECTION_IS_EARLY set.
> >
> > Normal hotplug memory will never have MEMBLOCK_NOMAP set in their memblock
> > regions. Because the flag MEMBLOCK_NOMAP was specifically designed and set
> > for firmware reserved memory regions. memblock_is_map_memory() can just be
> > skipped as its always going to be positive and that will be an optimization
> > for the normal hotplug memory. Like ZONE_DEVIE based memory, all hotplugged
> > normal memory too will not have SECTION_IS_EARLY set for their sections.
> >
> > Skipping memblock_is_map_memory() for all non early memory sections would
> > fix pfn_valid() problem for ZONE_DEVICE based memory and also improve its
> > performance for normal hotplug memory as well.
> >
> > Cc: Catalin Marinas <catalin.marinas@arm.com>
> > Cc: Will Deacon <will@kernel.org>
> > Cc: Ard Biesheuvel <ardb@kernel.org>
> > Cc: Robin Murphy <robin.murphy@arm.com>
> > Cc: linux-arm-kernel@lists.infradead.org
> > Cc: linux-kernel@vger.kernel.org
> > Fixes: 73b20c84d42d ("arm64: mm: implement pte_devmap support")
> > Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>
> Hello David/Mike,
>
> Given that we would need to rework early sections, memblock semantics via a
> new config i.e EARLY_SECTION_MEMMAP_HOLES and also some possible changes to
> ARCH_KEEP_MEMBLOCK and HAVE_ARCH_PFN_VALID, wondering if these patches here
> which fixes a problem (and improves performance) can be merged first. After
> that, I could start working on the proposed rework. Could you please let me
> know your thoughts on this. Thank you.

I didn't object to these patches, I think they are fine.
I agree that we can look into update of arm64's pfn_valid(), maybe right
after decrease of section size lands in.

> - Anshuman

--
Sincerely yours,
Mike.