To avoid bugs in userspace, we require that userspace provide UFFD_FEATURE_EXACT_ADDRESS when using UFFD_FEATURE_MINOR_HUGETLBFS_HGM, otherwise UFFDIO_API will fail with EINVAL. The potential confusion is this: without EXACT_ADDRESS, the address given in the userfaultfd message will be rounded down to the hugepage size. Userspace may think that, because they're using HGM, just UFFDIO_CONTINUE the interval [address, address+PAGE_SIZE), but for faults that didn't occur in the first base page of the hugepage, this won't resolve the fault. The only choice it has in this scenario is to UFFDIO_CONTINUE the interval [address, address+hugepage_size), which negates the purpose of using HGM in the first place. By requiring userspace to provide UFFD_FEATURE_EXACT_ADDRESS, there is no rounding, and userspace now has the information it needs to appropriately resolve the fault. Another potential solution here is to change the behavior when UFFD_FEATURE_EXACT_ADDRESS is not provided: when HGM is enabled, start rounding to PAGE_SIZE instead of to the hugepage size. I think requiring UFFD_FEATURE_EXACT_ADDRESS is cleaner. Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx> --- fs/userfaultfd.c | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index 0204108e3882..c8f21f53e37d 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -1990,6 +1990,17 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx, ~(UFFD_FEATURE_MINOR_HUGETLBFS | UFFD_FEATURE_MINOR_SHMEM); #ifndef CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING uffdio_api.features &= ~UFFD_FEATURE_MINOR_HUGETLBFS_HGM; +#else + + ret = -EINVAL; + if ((uffdio_api.features & UFFD_FEATURE_MINOR_HUGETLBFS_HGM) && + !(uffdio_api.features & UFFD_FEATURE_EXACT_ADDRESS)) + /* + * UFFD_FEATURE_MINOR_HUGETLBFS_HGM is mostly + * useless without UFFD_FEATURE_EXACT_ADDRESS, + * so require userspace to provide both. + */ + goto err_out; #endif /* CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING */ #endif /* CONFIG_HAVE_ARCH_USERFAULTFD_MINOR */ -- 2.38.0.135.g90850a2211-goog
