On 1/11/21 3:08 PM, Peter Xu wrote: > On Mon, Jan 11, 2021 at 02:42:48PM -0800, Mike Kravetz wrote: >> On 1/7/21 11:04 AM, Axel Rasmussen wrote: >>> Overview >>> ======== >>> >>> This series adds a new userfaultfd registration mode, >>> UFFDIO_REGISTER_MODE_MINOR. This allows userspace to intercept "minor" faults. >>> By "minor" fault, I mean the following situation: >>> >>> Let there exist two mappings (i.e., VMAs) to the same page(s) (shared memory). >>> One of the mappings is registered with userfaultfd (in minor mode), and the >>> other is not. Via the non-UFFD mapping, the underlying pages have already been >>> allocated & filled with some contents. The UFFD mapping has not yet been >>> faulted in; when it is touched for the first time, this results in what I'm >>> calling a "minor" fault. As a concrete example, when working with hugetlbfs, we >>> have huge_pte_none(), but find_lock_page() finds an existing page. >>> >>> We also add a new ioctl to resolve such faults: UFFDIO_CONTINUE. The idea is, >>> userspace resolves the fault by either a) doing nothing if the contents are >>> already correct, or b) updating the underlying contents using the second, >>> non-UFFD mapping (via memcpy/memset or similar, or something fancier like RDMA, >>> or etc...). In either case, userspace issues UFFDIO_CONTINUE to tell the kernel >>> "I have ensured the page contents are correct, carry on setting up the mapping". >>> >> >> One quick thought. >> >> This is not going to work as expected with hugetlbfs pmd sharing. If you >> are not familiar with hugetlbfs pmd sharing, you are not alone. :) >> >> pmd sharing is enabled for x86 and arm64 architectures. If there are multiple >> shared mappings of the same underlying hugetlbfs file or shared memory segment >> that are 'suitably aligned', then the PMD pages associated with those regions >> are shared by all the mappings. Suitably aligned means 'on a 1GB boundary' >> and 1GB in size. >> >> When pmds are shared, your mappings will never see a 'minor fault'. This >> is because the PMD (page table entries) is shared. > > Thanks for raising this, Mike. > > I've got a few patches that plan to disable huge pmd sharing for uffd in > general, e.g.: > > https://github.com/xzpeter/linux/commit/f9123e803d9bdd91bf6ef23b028087676bed1540 > https://github.com/xzpeter/linux/commit/aa9aeb5c4222a2fdb48793cdbc22902288454a31 > > I believe we don't want that for missing mode too, but it's just not extremely > important for missing mode yet, because in missing mode we normally monitor all > the processes that will be using the registered mm range. For example, in QEMU > postcopy migration with vhost-user hugetlbfs files as backends, we'll monitor > both the QEMU process and the DPDK program, so that either of the programs will > trigger a missing fault even if pmd shared between them. However again I think > it's not ideal since uffd (even if missing mode) is pgtable-based, so sharing > could always be too tricky. > > They're not yet posted to public yet since that's part of uffd-wp support for > hugetlbfs (along with shmem). So just raise this up to avoid potential > duplicated work before I post the patchset. > > (Will read into details soon; probably too many things piled up...) Thanks for the heads up about this Peter. I know Oracle DB really wants shared pmds -and- UFFD. I need to get details of their exact usage model. I know they primarily use SIGBUS, but use MISSING_HUGETLBFS as well. We may need to be more selective in when to disable. -- Mike Kravetz
