mm/rmap: fix old bug: munlocking THP missed other mlocks

The kernel recovers in due course from missing Mlocked pages: but there
was no point in calling page_mlock() (formerly known as
try_to_munlock()) on a THP, because nothing got done even when it was
found to be mapped in another VM_LOCKED vma.

It's true that we need to be careful: Mlocked accounting of pte-mapped
THPs is too difficult (so consistently avoided); but Mlocked accounting
of only-pmd-mapped THPs is supposed to work, even when multiple mappings
are mlocked and munlocked or munmapped.  Refine the tests.

There is already a VM_BUG_ON_PAGE(PageDoubleMap) in page_mlock(), so
page_mlock_one() does not even have to worry about that complication.

(I said the kernel recovers: but would page reclaim be likely to split
THP before rediscovering that it's VM_LOCKED? I've not followed that up)

Fixes: 9a73f61b

 ("thp, mlock: do not mlock PTE-mapped file huge pages")
Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Shakeel Butt <shakeelb...

mm/rmap.c

@@ -1442,8 +1442,9 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
 		 */
 		if (!(flags & TTU_IGNORE_MLOCK)) {
 			if (vma->vm_flags & VM_LOCKED) {
-				/* PTE-mapped THP are never mlocked */
-				if (!PageTransCompound(page)) {
+				/* PTE-mapped THP are never marked as mlocked */
+				if (!PageTransCompound(page) ||
+				    (PageHead(page) && !PageDoubleMap(page))) {
 					/*
 					 * Holding pte lock, we do *not* need
 					 * mmap_lock here
@@ -1984,9 +1985,11 @@ static bool page_mlock_one(struct page *page, struct vm_area_struct *vma,
 		 * munlock_vma_pages_range().
 		 */
 		if (vma->vm_flags & VM_LOCKED) {
-			/* PTE-mapped THP are never mlocked */
-			if (!PageTransCompound(page))
-				mlock_vma_page(page);
+			/*
+			 * PTE-mapped THP are never marked as mlocked, but
+			 * this function is never called when PageDoubleMap().
+			 */
+			mlock_vma_page(page);
 			page_vma_mapped_walk_done(&pvmw);
 		}