mm: hugetlb_vmemmap: simplify reset_struct_pages()

We can choose to copy three contiguous tail pages' content to the first
three pages instead of copying one by one to simplify the code and reduce
code size from 229 bytes to 63 bytes.  The BUILD_BUG_ON() aims to avoid
out-of-bounds accesses.

Link: https://lkml.kernel.org/r/20220819035532.6189-1-songmuchun@bytedance.comSigned-off-by: Muchun Song <songmuchun@bytedance.com>
Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/hugetlb_vmemmap.c

@@ -265,11 +265,10 @@ static void vmemmap_remap_pte(pte_t *pte, unsigned long addr,
 
 static inline void reset_struct_pages(struct page *start)
 {
-	int i;
 	struct page *from = start + NR_RESET_STRUCT_PAGE;
 
-	for (i = 0; i < NR_RESET_STRUCT_PAGE; i++)
-		memcpy(start + i, from, sizeof(*from));
+	BUILD_BUG_ON(NR_RESET_STRUCT_PAGE * 2 > PAGE_SIZE / sizeof(struct page));
+	memcpy(start, from, sizeof(*from) * NR_RESET_STRUCT_PAGE);
 }
 
 static void vmemmap_restore_pte(pte_t *pte, unsigned long addr,