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

[Anshuman Khandual]

On 3/4/21 2:54 AM, Will Deacon wrote:
> On Wed, Mar 03, 2021 at 07:04:33PM +0000, 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@xxxxxxxxxx
>>>>
>>>> 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().
>>
>> 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.
>>
>> Another situation could be that pfn_valid() returns true when no memory
>> is mapped for that pfn.
> 
> The case I wondered about was __pfn_to_section() with a bogus pfn, since
> with patch 2/2 we call that *before* checking that pfn_to_section_nr() is
> sane.

Right, that is problematic. __pfn_to_section() should not be called without
first validating pfn_to_section_nr(), as it could cause out-of-bound access
on mem_section buffer. Will fix that order but as there is no test scenario
which is definitive for this reported regression, how should we ensure that
it fixes the problem ?