On Wed, Jan 04, 2023 at 11:19:10PM +0000, James Houghton wrote: > PMD sharing can only be done in PUD_SIZE-aligned pieces of VMAs; > however, it is possible that HugeTLB VMAs are split without unsharing > the PMDs first. > > Without this fix, it is possible to hit the uffd-wp-related WARN_ON_ONCE > in hugetlb_change_protection [1]. The key there is that > hugetlb_unshare_all_pmds will not attempt to unshare PMDs in > non-PUD_SIZE-aligned sections of the VMA. > > It might seem ideal to unshare in hugetlb_vm_op_open, but we need to > unshare in both the new and old VMAs, so unsharing in > hugetlb_vm_op_split seems natural. > > [1]: https://lore.kernel.org/linux-mm/CADrL8HVeOkj0QH5VZZbRzybNE8CG-tEGFshnA+bG9nMgcWtBSg@xxxxxxxxxxxxxx/ > > Fixes: 6dfeaff93be1 ("hugetlb/userfaultfd: unshare all pmds for hugetlbfs when register wp") > Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx> Acked-by: Peter Xu <peterx@xxxxxxxxxx> Thanks, -- Peter Xu
