mm/madvise: fix potential pte_unmap_unlock pte error

We can't assume pte_offset_map_lock will return same orig_pte value. So
it's necessary to reacquire the orig_pte or pte_unmap_unlock will unmap
the stale pte.

Link: https://lkml.kernel.org/r/20220416081416.23304-1-linmiaohe@huawei.com
Fixes: 9c276cc6 ("mm: introduce MADV_COLD")
Fixes: 854e9ed0 ("mm: support madvise(MADV_FREE)")
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/madvise.c

@@ -432,12 +432,12 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
 			if (split_huge_page(page)) {
 				unlock_page(page);
 				put_page(page);
-				pte_offset_map_lock(mm, pmd, addr, &ptl);
+				orig_pte = pte_offset_map_lock(mm, pmd, addr, &ptl);
 				break;
 			}
 			unlock_page(page);
 			put_page(page);
-			pte = pte_offset_map_lock(mm, pmd, addr, &ptl);
+			orig_pte = pte = pte_offset_map_lock(mm, pmd, addr, &ptl);
 			pte--;
 			addr -= PAGE_SIZE;
 			continue;
@@ -648,12 +648,12 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
 			if (split_huge_page(page)) {
 				unlock_page(page);
 				put_page(page);
-				pte_offset_map_lock(mm, pmd, addr, &ptl);
+				orig_pte = pte_offset_map_lock(mm, pmd, addr, &ptl);
 				goto out;
 			}
 			unlock_page(page);
 			put_page(page);
-			pte = pte_offset_map_lock(mm, pmd, addr, &ptl);
+			orig_pte = pte = pte_offset_map_lock(mm, pmd, addr, &ptl);
 			pte--;
 			addr -= PAGE_SIZE;
 			continue;