Re: [PATCH V2 1/2] arm64/mm: Fix pfn_valid() for ZONE_DEVICE based memory

[David Hildenbrand <david@redhat.com>]

On 02.02.21 13:35, Will Deacon wrote:
> On Tue, Feb 02, 2021 at 12:32:15PM +0000, Will Deacon wrote:
>> On Tue, Feb 02, 2021 at 09:41:53AM +0530, Anshuman Khandual wrote:
>>> pfn_valid() validates a pfn but basically it checks for a valid struct page
>>> backing for that pfn. It should always return positive for memory ranges
>>> backed with struct page mapping. But currently pfn_valid() fails for all
>>> ZONE_DEVICE based memory types even though they have struct page mapping.
>>>
>>> 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_DEVICE based memory, all normal
>>> hotplugged 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.
>>
>> Hmm. Although I follow your logic, this does seem to rely on an awful lot of
>> assumptions to continue to hold true as the kernel evolves. In particular,
>> how do we ensure that early sections are always fully backed with
> 
> Sorry, typo here:       ^^^ should be *non-early* sections.

It might be a good idea to have a look at generic 
include/linux/mmzone.h:pfn_valid()

As I expressed already, long term we should really get rid of the arm64 
variant and rather special-case the generic one. Then we won't go out of 
sync - just as it happened with ZONE_DEVICE handling here.

-- 
Thanks,

David / dhildenb