On 05.01.23 19:01, Nadav Amit wrote:
On Jan 5, 2023, at 12:59 AM, David Hildenbrand <david@xxxxxxxxxx> wrote: On 05.01.23 04:10, Nadav Amit wrote:On Jan 4, 2023, at 2:52 PM, Peter Xu <peterx@xxxxxxxxxx> 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
