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    SubjectRe: [PATCH 1/3] mm/hugetlb: Pre-allocate pgtable pages for uffd wr-protects
    On 01/04/23 17:52, 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(-)

    Thanks Peter and James!

    Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
    --
    Mike Kravetz

    >
    > 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) {
    > - 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)) {
    > --
    > 2.37.3
    >

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