Messages in this thread | | | Date | Mon, 9 Jan 2023 09:36:47 +0100 | Subject | Re: [PATCH 3/3] mm/uffd: Detect pgtable allocation failures | From | David Hildenbrand <> |
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On 05.01.23 19:01, Nadav Amit wrote: > > >> On Jan 5, 2023, at 12:59 AM, David Hildenbrand <david@redhat.com> wrote: >> >> On 05.01.23 04:10, Nadav Amit wrote: >>>> On Jan 4, 2023, at 2:52 PM, Peter Xu <peterx@redhat.com> wrote: >>>> >>>> Before this patch, when there's any pgtable allocation issues happened >>>> during change_protection(), the error will be ignored from the syscall. >>>> For shmem, there will be an error dumped into the host dmesg. Two issues >>>> with that: >>>> >>>> (1) Doing a trace dump when allocation fails is not anything close to >>>> grace.. >>>> >>>> (2) The user should be notified with any kind of such error, so the user >>>> can trap it and decide what to do next, either by retrying, or stop >>>> the process properly, or anything else. >>>> >>>> For userfault users, this will change the API of UFFDIO_WRITEPROTECT when >>>> pgtable allocation failure happened. It should not normally break anyone, >>>> though. If it breaks, then in good ways. >>>> >>>> One man-page update will be on the way to introduce the new -ENOMEM for >>>> UFFDIO_WRITEPROTECT. Not marking stable so we keep the old behavior on the >>>> 5.19-till-now kernels. >>> I understand that the current assumption is that change_protection() should >>> fully succeed or fail, and I guess this is the current behavior. >>> However, to be more “future-proof” perhaps this needs to be revisited. >>> For instance, UFFDIO_WRITEPROTECT can benefit from the ability to (based on >>> userspace request) prevent write-protection of pages that are pinned. This is >>> necessary to allow userspace uffd monitor to avoid write-protection of >>> O_DIRECT’d memory, for instance, that might change even if a uffd monitor >>> considers it write-protected. >> >> Just a note that this is pretty tricky IMHO, because: >> >> a) We cannot distinguished "pinned readable" from "pinned writable" >> b) We can have false positives ("pinned") even for compound pages due to >> concurrent GUP-fast. >> c) Synchronizing against GUP-fast is pretty tricky ... as we learned. >> Concurrent pinning is usually problematic. >> d) O_DIRECT still uses FOLL_GET and we cannot identify that. (at least >> that should be figured out at one point) > > My prototype used the page-count IIRC, so it had false-positives (but
I suspect GUP-fast is still problematic, I might be wrong.
> addressed O_DIRECT). And yes, precise refinement is complicated. However, > if you need to uffd-wp memory, then without such a mechanism you need to > ensure no kerenl/DMA write to these pages is possible. The only other > option I can think of is interposing/seccomp on a variety of syscalls, > to prevent uffd-wp of such memory.
The whole thing reminds me of MADV_DONTNEED+pinning: an application shouldn't do it, because you can only get it wrong :) I know, that's a bad answer.
-- Thanks,
David / dhildenb
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