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:

quoted

On Tue 14-05-19 14:30:05, Anshuman Khandual wrote:

quoted

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.

quoted

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?

quoted

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

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