mm/huge_memory: comment the subtly logic in __split_huge_pmd

It's dangerous and wrong to call page_folio(pmd_page(*pmd)) when pmd isn't
present. But the caller guarantees pmd is present when folio is set. So we
should be safe here. Add comment to make it clear.

Link: https://lkml.kernel.org/r/20220704132201.14611-16-linmiaohe@huawei.comSigned-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Zach O'Keefe <zokeefe@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/huge_memory.c

@@ -2235,6 +2235,10 @@ void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
 
 	if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) ||
 	    is_pmd_migration_entry(*pmd)) {
+		/*
+		 * It's safe to call pmd_page when folio is set because it's
+		 * guaranteed that pmd is present.
+		 */
 		if (folio && folio != page_folio(pmd_page(*pmd)))
 			goto out;
 		__split_huge_pmd_locked(vma, pmd, range.start, freeze);