Re: [PATCH 1/1] userfaultfd: fix move_pages_pte() splitting folio under RCU read lock

On Fri, Dec 29, 2023 at 06:56:07PM -0800, Suren Baghdasaryan wrote:

@@ -1078,9 +1078,14 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, /* at this point we have src_folio locked */ if (folio_test_large(src_folio)) {

• 	/* split_folio() can block */
  

• 	pte_unmap(&orig_src_pte);
  

• 	pte_unmap(&orig_dst_pte);
  

• 	src_pte = dst_pte = NULL;
  err = split_folio(src_folio);
  if (err)
  	goto out;
  

• 	goto retry;
  

  }

Do we also need to clear src_folio and src_folio_pte? If the folio is a thp, I think it means it's pte mapped here. Then after the split we may want to fetch the small folio after the split, not the head one?