Re: [PATCH v2] hugetlb: unshare some PMDs when splitting VMAs

[Peter Xu]

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