/*
* mmap_lock must be held on entry.  If @locked != NULL and *@flags
* does not include FOLL_NOWAIT, the mmap_lock may be released.  If it
* is, *@locked will be set to 0 and -EBUSY returned.
*/
static int faultin_page(struct vm_area_struct *vma,
	unsigned long address, unsigned int *flags, bool unshare,
	int *locked)
{
  ...

  /*
  * The VM_FAULT_WRITE bit tells us that do_wp_page has broken COW when
  * necessary, even if maybe_mkwrite decided not to set pte_write. We
  * can thus safely do subsequent page lookups as if they were reads.
  * But only do so when looping for pte_write is futile: in some cases
  * userspace may also be wanting to write to the gotten user page,
  * which a read fault here might prevent (a readonly page might get
  * reCOWed by userspace write).
  */
  if ((ret & VM_FAULT_WRITE) && !(vma->vm_flags & VM_WRITE))
    *flags |= FOLL_COW;
  return 0;
}

static long __get_user_pages(struct mm_struct *mm,
 		unsigned long start, unsigned long nr_pages,
 		unsigned int gup_flags, struct page **pages,
 		struct vm_area_struct **vmas, int *locked)
 {
 	...
 retry:
 		/*
 		 * If we have a pending SIGKILL, don't keep faulting pages and
 		 * potentially allocating memory.
 		 */
 		if (fatal_signal_pending(current)) {
 			ret = -EINTR;
 			goto out;
 		}
 		cond_resched();

 		page = follow_page_mask(vma, start, foll_flags, &ctx); // can_follow_write_pte
 		if (!page || PTR_ERR(page) == -EMLINK) {
 			ret = faultin_page(vma, start, &foll_flags,
 					   PTR_ERR(page) == -EMLINK, locked); // flags |= FOLL_COW
 			switch (ret) {
 			case 0:
 				goto retry; // try follow page again
 			case -EBUSY:
 			case -EAGAIN:
 				ret = 0;
 				fallthrough;
 			case -EFAULT:
 			case -ENOMEM:
 			case -EHWPOISON:
 				goto out;
 			}
 			BUG();
 		} else if (PTR_ERR(page) == -EEXIST) {
    ...
 }

/*
* Install PTEs, to map dst_addr (within dst_vma) to page.
*
* This function handles both MCOPY_ATOMIC_NORMAL and _CONTINUE for both shmem
* and anon, and for both shared and private VMAs.
*/
int mfill_atomic_install_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
		     struct vm_area_struct *dst_vma,
		     unsigned long dst_addr, struct page *page,
		     bool newly_allocated, bool wp_copy)
{
 	...

 	_dst_pte = mk_pte(page, dst_vma->vm_page_prot);
 	_dst_pte = pte_mkdirty(_dst_pte); // set pte dirty unconditionally
 	if (page_in_cache && !vm_shared)
 		writable = false;

 	...
}