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

[Michal Hocko]

On Thu 16-05-19 11:23:54, 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.

OK, I see. Thanks for the clarification.

> 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.

While I agree, do we rather want to document that you shouldn't be using
the debugging tool while the hotplug is ongoing because you might get a
garbage or crash the kernel in the worst case? In other words is the
absolute correctness worth the additional maint. burden wrt. to future
hotplug changes?

> > 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()?

You have to pin the page to make sure the hotplug is not going to
offline it.

> The big problem for the ptdump code is when tables are freed, since the
> pages can be reused elsewhere (or hot-removed), causing the ptdump code
> to explode.

Yes, I see the danger. I am just wondering whether living with that is
reasonable considering this is a debugfs code.
-- 
Michal Hocko
SUSE Labs