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

[David Hildenbrand <david@redhat.com>]

On 02.02.21 13:51, Will Deacon wrote:
> On Tue, Feb 02, 2021 at 01:39:29PM +0100, David Hildenbrand wrote:
>> 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()
> 
> The generic implementation already makes assumptions that aren't true on
> arm64, so that's why we've ended up with our own implementation. But the
> patches here put us in a position where I worry that pfn_valid() may return
> 'true' in future for cases where the underlying struct page is either
> non-existent or bogus, and debugging those failures really sucks. We had a
> raft of those back when NOMAP was introduced and I don't want to re-live
> that experience.

Yeah, and I agree when it comes to boot mem. However, the way generic 
memory hotplug/memremap infrastructure (->!early sections) works does 
not allow for such special cases you mention and would break quite some 
other code if messed up. So I wouldn't worry about that part too much 
for now.

> 
>> 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.
> 
> Why does this have to be long term? This ZONE_DEVICE stuff could be the
> carrot on the stick :)

Yes, I suggested to do it now, but Anshuman convinced me that doing a 
simple fix upfront might be cleaner --- for example when it comes to 
backporting :)

-- 
Thanks,

David / dhildenb