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SubjectRe: [PATCH 1/3] mm/hugetlb: Pre-allocate pgtable pages for uffd wr-protects
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On 04.01.23 23:52, Peter Xu wrote:
> Userfaultfd-wp uses pte markers to mark wr-protected pages for both shmem
> and hugetlb. Shmem has pre-allocation ready for markers, but hugetlb path
> was overlooked.
>
> Doing so by calling huge_pte_alloc() if the initial pgtable walk fails to
> find the huge ptep. It's possible that huge_pte_alloc() can fail with high
> memory pressure, in that case stop the loop immediately and fail silently.
> This is not the most ideal solution but it matches with what we do with
> shmem meanwhile it avoids the splat in dmesg.
>
> Cc: linux-stable <stable@vger.kernel.org> # 5.19+
> Fixes: 60dfaad65aa9 ("mm/hugetlb: allow uffd wr-protect none ptes")
> Reported-by: James Houghton <jthoughton@google.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
> mm/hugetlb.c | 13 +++++++++++--
> 1 file changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index bf7a1f628357..017d9159cddf 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6649,8 +6649,17 @@ unsigned long hugetlb_change_protection(struct vm_area_struct *vma,
> spinlock_t *ptl;
> ptep = hugetlb_walk(vma, address, psize);

if (!ptep && likely(!uffd_wp)) {
/* Nothing to protect. */
address |= last_addr_mask;
continue;
} else if (!ptep) {
...
}

Might look slightly more readable would minimize changes. This should
work, so

Acked-by: David Hildenbrand <david@redhat.com>

> if (!ptep) {
> - address |= last_addr_mask;
> - continue;
> + if (!uffd_wp) {
> + address |= last_addr_mask;
> + continue;
> + }
> + /*
> + * Userfaultfd wr-protect requires pgtable
> + * pre-allocations to install pte markers.
> + */
> + ptep = huge_pte_alloc(mm, vma, address, psize);
> + if (!ptep)
> + break;
> }
> ptl = huge_pte_lock(h, mm, ptep);
> if (huge_pmd_unshare(mm, vma, address, ptep)) {

--
Thanks,

David / dhildenb

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