[PATCH 2/4] mm/rmap: fix old bug: munlocking THP missed other mlocks

Hugh Dickins
2021-07-07T20:09:03Z

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: 9a73f61bdb8a ("thp, mlock: do not mlock PTE-mapped file huge pages")
Signed-off-by: Hugh Dickins <hughd@google.com>
---
mm/rmap.c | 13 ++++++++-----
1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 746013e282c3..0e83c3be8568 100644
--- a/mm/rmap.c
+++ b/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);
}

--
2.26.2