Messages in this thread | | | Date | Fri, 19 May 2023 10:38:29 +0200 | From | David Hildenbrand <> | Subject | Re: [PATCH 1/3] mm: userfaultfd: add new UFFDIO_SIGBUS ioctl |
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On 18.05.23 22:38, Axel Rasmussen wrote: > On Thu, May 18, 2023 at 9:05 AM Peter Xu <peterx@redhat.com> wrote: >> >> On Wed, May 17, 2023 at 05:43:53PM -0700, Jiaqi Yan wrote: >>> On Wed, May 17, 2023 at 3:29 PM Axel Rasmussen <axelrasmussen@google.com> wrote: >>>> >>>> On Wed, May 17, 2023 at 3:20 PM Peter Xu <peterx@redhat.com> wrote: >>>>> >>>>> On Wed, May 17, 2023 at 06:12:33PM -0400, Peter Xu wrote: >>>>>> On Thu, May 11, 2023 at 03:00:09PM -0700, James Houghton wrote: >>>>>>> On Thu, May 11, 2023 at 11:24 AM Axel Rasmussen >>>>>>> <axelrasmussen@google.com> wrote: >>>>>>>> >>>>>>>> So the basic way to use this new feature is: >>>>>>>> >>>>>>>> - On the new host, the guest's memory is registered with userfaultfd, in >>>>>>>> either MISSING or MINOR mode (doesn't really matter for this purpose). >>>>>>>> - On any first access, we get a userfaultfd event. At this point we can >>>>>>>> communicate with the old host to find out if the page was poisoned. >>>>>>>> - If so, we can respond with a UFFDIO_SIGBUS - this places a swap marker >>>>>>>> so any future accesses will SIGBUS. Because the pte is now "present", >>>>>>>> future accesses won't generate more userfaultfd events, they'll just >>>>>>>> SIGBUS directly. >>>>>>> >>>>>>> I want to clarify the SIGBUS mechanism here when KVM is involved, >>>>>>> keeping in mind that we need to be able to inject an MCE into the >>>>>>> guest for this to be useful. >>>>>>> >>>>>>> 1. vCPU gets an EPT violation --> KVM attempts GUP. >>>>>>> 2. GUP finds a PTE_MARKER_UFFD_SIGBUS and returns VM_FAULT_SIGBUS. >>>>>>> 3. KVM finds that GUP failed and returns -EFAULT. >>>>>>> >>>>>>> This is different than if GUP found poison, in which case KVM will >>>>>>> actually queue up a SIGBUS *containing the address of the fault*, and >>>>>>> userspace can use it to inject an appropriate MCE into the guest. With >>>>>>> UFFDIO_SIGBUS, we are missing the address! >>>>>>> >>>>>>> I see three options: >>>>>>> 1. Make KVM_RUN queue up a signal for any VM_FAULT_SIGBUS. I think >>>>>>> this is pointless. >>>>>>> 2. Don't have UFFDIO_SIGBUS install a PTE entry, but instead have a >>>>>>> UFFDIO_WAKE_MODE_SIGBUS, where upon waking, we return VM_FAULT_SIGBUS >>>>>>> instead of VM_FAULT_RETRY. We will keep getting userfaults on repeated >>>>>>> accesses, just like how we get repeated signals for real poison. >>>>>>> 3. Use this in conjunction with the additional KVM EFAULT info that >>>>>>> Anish proposed (the first part of [1]). >>>>>>> >>>>>>> I think option 3 is fine. :) >>>>>> >>>>>> Or... option 4) just to use either MADV_HWPOISON or hwpoison-inject? :) >>>>> >>>>> I just remember Axel mentioned this in the commit message, and just in case >>>>> this is why option 4) was ruled out: >>>>> >>>>> They expect that once poisoned, pages can never become >>>>> "un-poisoned". So, when we live migrate the VM, we need to preserve >>>>> the poisoned status of these pages. >>>>> >>>>> Just to supplement on this point: we do have unpoison (echoing to >>>>> "debug/hwpoison/hwpoison_unpoison"), or am I wrong? >>> >>> If I read unpoison_memory() correctly, once there is a real hardware >>> memory corruption (hw_memory_failure will be set), unpoison will stop >>> working and return EOPNOTSUPP. >>> >>> I know some cloud providers evacuating VMs once a single memory error >>> happens, so not supporting unpoison is probably not a big deal for >>> them. BUT others do keep VM running until more errors show up later, >>> which could be long after the 1st error. >> >> We're talking about postcopy migrating a VM has poisoned page on src, >> rather than on dst host, am I right? IOW, the dest hwpoison should be >> fake. >> >> If so, then I would assume that's the case where all the pages on the dest >> host is still all good (so hw_memory_failure not yet set, or I doubt the >> judgement of being a migration target after all)? >> >> The other thing is even if dest host has hw poisoned page, I'm not sure >> whether hw_memory_failure is the only way to solve this. >> >> I saw that this is something got worked on before from Zhenwei, David used >> to have some reasoning on why it was suggested like using a global knob: >> >> https://lore.kernel.org/all/d7927214-e433-c26d-7a9c-a291ced81887@redhat.com/ >> >> Two major issues here afaics: >> >> - Zhenwei's approach only considered x86 hwpoison - it relies on kpte >> having !present in entries but that's x86 specific rather than generic >> to memory_failure.c. >> >> - It is _assumed_ that hwpoison injection is for debugging only. >> >> I'm not sure whether you can fix 1) by some other ways, e.g., what if the >> host just remember all the hardware poisoned pfns (or remember >> soft-poisoned ones, but then here we need to be careful on removing them >> from the list when it's hwpoisoned for real)? It sounds like there's >> opportunity on providing a generic solution rather than relying on >> !pte_present(). >> >> For 2) IMHO that's not a big issue, you can declare it'll be used in !debug >> but production systems so as to boost the feature importance with a real >> use case. >> >> So far I'd say it'll be great to leverage what it's already there in linux >> and make it as generic as possible. The only issue is probably >> CAP_ADMIN... not sure whether we can have some way to provide !ADMIN >> somehow, or you can simply work around this issue. > > As you mention below I think the key distinction is the scope - I > think MADV_HWPOISON affects the whole system, including other > processes. > > For our purposes, we really just want to "poison" this particular > virtual address (the HVA, from the VM's perspective), not even other > mappings of the same shared memory. I think that behavior is different > from MADV_HWPOISON, at least.
MADV_HWPOISON really is the wrong interface to use. See "man madvise".
We don't want to allow arbitrary users to hwpoison+offline absolutely healthy physical memory, which is what MADV_HWPOISON is all about.
As you say, we want to turn an unpopulated (!present) virtual address to mimic like we had a MCE on a page that would have been previously mapped here: install a hwpoison marker without actually poisoning any present page. In fact, we'd even want to fail if there *is* something mapped.
Sure, one could teach MADV_HWPOISON to allow unprivileged users to do that for !present PTE entries, and fail for unprivileged users if there is a present PTE entry. I'm not sure if that's the cleanest approach, though, and a new MADV as suggested in this thread would eventually be cleaner.
-- Thanks,
David / dhildenb
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