Messages in this thread |  | | Date | Thu, 16 May 2019 13:05:29 +0200 | From | Michal Hocko <> | Subject | Re: [PATCH V3 2/4] arm64/mm: Hold memory hotplug lock while walking for kernel page table dump |
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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
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