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

[David Hildenbrand <david@redhat.com>]

On 03.03.21 20:04, Catalin Marinas wrote:
> On Thu, Feb 11, 2021 at 01:35:56PM +0100, David Hildenbrand wrote:
>> On 11.02.21 13:10, Anshuman Khandual wrote:
>>> On 2/11/21 5:23 PM, Will Deacon wrote:
>>>> ... 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).
> 
> I couldn't figure out how this could fail with Anshuman's patches.
> Will's suspicion is that some invalid/null pointer gets dereferenced
> before being initialised but the only case I see is somewhere in
> pfn_section_valid() (ms->usage) if valid_section() && !early_section().

Indeed, it looks like a latent bug.

> 
> Assuming that we do get a valid_section(ms) && !early_section(ms), is
> there a case where ms->usage is not initialised? I guess races with
> section_deactivate() are not possible this early.
> 

Do you have access to that machine? We could identify which path is 
taken quite easily.

> Another situation could be that pfn_valid() returns true when no memory
> is mapped for that pfn.
> 
>> 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.
> 
> It is indeed very early, we can't even get the early console output.

So even before any hotplug really happens. All sections should be early 
at that point I guess.

> Debugging this is even harder as it's only misbehaving on a board we
> don't have access to.
> 
> On the logic in this patch, is the hot-added memory always covering a
> full subsection? The arm64 pfn_valid() currently relies on
> memblock_is_map_memory() but the patch changes it to
> pfn_section_valid(). So if hot-added memory doesn't cover the full
> subsection, it may return true even if the pfn is not mapped.

Hotplugged System RAM always covers full sections. Hotplugged 
ZONE_DEVICE always covers full subsections. pfn_section_valid() properly 
handles both cases. (see generic pfn_valid())

-- 
Thanks,

David / dhildenb