Re: [PATCH V3 2/4] arm64/mm: Hold memory hotplug lock while walking for kernel page table dump

[David Hildenbrand]

On 16.05.19 13:16, Michal Hocko wrote:
> On Thu 16-05-19 16:36:12, Anshuman Khandual wrote:
>> On 05/16/2019 03:53 PM, Mark Rutland wrote:
>>> Hi Michal,
>>>
>>> On Wed, May 15, 2019 at 06:58:47PM +0200, Michal Hocko wrote:
>>>> On Tue 14-05-19 14:30:05, Anshuman Khandual wrote:
>>>>> The arm64 pagetable dump code can race with concurrent modification of the
>>>>> kernel page tables. When a leaf entries are modified concurrently, the dump
>>>>> code may log stale or inconsistent information for a VA range, but this is
>>>>> otherwise not harmful.
>>>>>
>>>>> When intermediate levels of table are freed, the dump code will continue to
>>>>> use memory which has been freed and potentially reallocated for another
>>>>> purpose. In such cases, the dump code may dereference bogus addressses,
>>>>> leading to a number of potential problems.
>>>>>
>>>>> Intermediate levels of table may by freed during memory hot-remove, or when
>>>>> installing a huge mapping in the vmalloc region. To avoid racing with these
>>>>> cases, take the memory hotplug lock when walking the kernel page table.
>>>>
>>>> Why is this a problem only on arm64 
>>>
>>> It looks like it's not -- I think we're just the first to realise this.
>>>
>>> AFAICT x86's debugfs ptdump has the same issue if run conccurently with
>>> memory hot remove. If 32-bit arm supported hot-remove, its ptdump code
>>> would have the same issue.
>>>
>>>> and why do we even care for debugfs? Does anybody rely on this thing
>>>> to be reliable? Do we even need it? Who is using the file?
>>>
>>> The debugfs part is used intermittently by a few people working on the
>>> arm64 kernel page tables. We use that both to sanity-check that kernel
>>> page tables are created/updated correctly after changes to the arm64 mmu
>>> code, and also to debug issues if/when we encounter issues that appear
>>> to be the result of kernel page table corruption.
>>>
>>> So while it's rare to need it, it's really useful to have when we do
>>> need it, and I'd rather not remove it. I'd also rather that it didn't
>>> have latent issues where we can accidentally crash the kernel when using
>>> it, which is what this patch is addressing.
>>>
>>>> I am asking because I would really love to make mem hotplug locking less
>>>> scattered outside of the core MM than more. Most users simply shouldn't
>>>> care. Pfn walkers should rely on pfn_to_online_page.
>>>
>>> I'm not sure if that would help us here; IIUC pfn_to_online_page() alone
>>> doesn't ensure that the page remains online. Is there a way to achieve
>>> that other than get_online_mems()?
>>
>> Still wondering how pfn_to_online_page() is applicable here. It validates
>> a given PFN and whether its online from sparse section mapping perspective
>> before giving it's struct page. IIUC it is used during a linear scanning
>> of a physical address range not for a page table walk. So how it can solve
>> the problem when a struct page which was used as an intermediate level page
>> table page gets released back to the buddy from another concurrent thread ?
> 
> Well, my comment about pfn_to_online_page was more generic and it might
> not apply to this specific case. I meant to say that the code outside of
> the core MM shouldn't really care about the hotplug locking.
> 

What am I missing, how is it guaranteed that a page doesn't get
offlined/removed without holding a lock here?

We would at least need some RCU mechnism or similar to sync against
pages vanishing.

pfn_to_online_page() assumes that somebody touches a page he doesn't
own. There has to be some way for core-mm to realize this and defer
offlining/removinf.

-- 

Thanks,

David / dhildenb