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

[David Hildenbrand <david@redhat.com>]

On 11.02.21 13:10, Anshuman Khandual wrote:
> 
> 
> On 2/11/21 5:23 PM, Will Deacon wrote:
>> On Fri, Feb 05, 2021 at 06:55:53PM +0000, Will Deacon wrote:
>>> On Wed, Feb 03, 2021 at 09:20:39AM +0530, Anshuman Khandual wrote:
>>>> On 2/2/21 6:26 PM, David Hildenbrand wrote:
>>>>> On 02.02.21 13:51, Will Deacon wrote:
>>>>>> On Tue, Feb 02, 2021 at 01:39:29PM +0100, David Hildenbrand wrote:
>>>>>>> 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 :)
>>>>
>>>> Right. The current pfn_valid() breaks for ZONE_DEVICE memory and this fixes
>>>> the problem in the present context which can be easily backported if required.
>>>>
>>>> Changing or rather overhauling the generic code with new configs as proposed
>>>> earlier (which I am planning to work on subsequently) would definitely be an
>>>> improvement for the current pfn_valid() situation in terms of maintainability
>>>> but then it should not stop us from fixing the problem now.
>>>
>>> Alright, I've mulled this over a bit. I don't agree that this patch helps
>>> with maintainability (quite the opposite, in fact), but perfection is the
>>> enemy of the good so I'll queue the series for 5.12. However, I'll revert
>>> the changes at the first sign of a problem, so please do work towards a
>>> generic solution which can replace this in the medium term.
>>
>> ... and dropped. These patches appear to be responsible for a boot
>> regression reported by CKI:
> 
> Ahh, boot regression ? These patches only change the behaviour
> for non boot memory only.
> 
>>
>> https://lore.kernel.org/r/cki.8D1CB60FEC.K6NJMEFQPV@redhat.com
> 
> Will look into the logs and see if there is something pointing to
> the problem.
> 

It's strange. One thing I can imagine is a mis-detection of early 
sections. However, I don't see that happening:

In sparse_init_nid(), we:
1. Initialize the memmap
2. Set SECTION_IS_EARLY | SECTION_HAS_MEM_MAP via
    sparse_init_one_section()

Only hotplugged sections (DIMMs, dax/kmem) set SECTION_HAS_MEM_MAP 
without SECTION_IS_EARLY - which is correct, because these are not early.

So once we know that we have valid_section() -- SECTION_HAS_MEM_MAP is 
set -- early_section() should be correct.

Even if someone would be doing a pfn_valid() after 
memblocks_present()->memory_present() but before
sparse_init_nid(), we should be fine (!valid_section() -> return 0).


As it happens early during boot, I doubt that some NVDIMMs that get 
detected and added early during boot as system RAM (via dax/kmem). Are 
the problem.

-- 
Thanks,

David / dhildenb