userfaultfd_must

#else
static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
                     struct vm_area_struct *vma,
                     unsigned long address,
                     unsigned long flags,
                     unsigned long reason)
{
    return false;   /* should never get here */
}
#endif /* CONFIG_HUGETLB_PAGE */
 
/*
 * Verify the pagetables are still not ok after having reigstered into
 * the fault_pending_wqh to avoid userland having to UFFDIO_WAKE any
 * userfault that has already been resolved, if userfaultfd_read and
 * UFFDIO_COPY|ZEROPAGE are being run simultaneously on two different
 * threads.
 */
static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
                     unsigned long address,
                     unsigned long flags,
                     unsigned long reason)
{
    struct mm_struct *mm = ctx->mm;
    pgd_t *pgd;
    p4d_t *p4d;
    pud_t *pud;
    pmd_t *pmd, _pmd;
    pte_t *pte;
    bool ret = true;
 
    VM_BUG_ON(!rwsem_is_locked(&mm->mmap_sem));
 
    pgd = pgd_offset(mm, address);
    if (!pgd_present(*pgd))
        goto out;
    p4d = p4d_offset(pgd, address);
    if (!p4d_present(*p4d))
        goto out;
    pud = pud_offset(p4d, address);
    if (!pud_present(*pud))
        goto out;
    pmd = pmd_offset(pud, address);
    /*
     * READ_ONCE must function as a barrier with narrower scope
     * and it must be equivalent to:
     *  _pmd = *pmd; barrier();
     *
     * This is to deal with the instability (as in
     * pmd_trans_unstable) of the pmd.
     */
    _pmd = READ_ONCE(*pmd);
    if (pmd_none(_pmd))
        goto out;
 
    ret = false;
    if (!pmd_present(_pmd))
        goto out;
 
    if (pmd_trans_huge(_pmd))
        goto out;
 
    /*
     * the pmd is stable (as in !pmd_trans_unstable) so we can re-read it
     * and use the standard pte_offset_map() instead of parsing _pmd.
     */
    pte = pte_offset_map(pmd, address);
    /*
     * Lockless access: we're in a wait_event so it's ok if it
     * changes under us.
     */
    if (pte_none(*pte))
        ret = true;
    pte_unmap(pte);
 
out:
    return ret;
}
函数源码
