[PATCH] mm: prevent droppable mappings from being locked

[Anthony Yznaga <anthony.yznaga@oracle.com>]

Mappings created with MAP_DROPPABLE cannot be locked via mlock() due
to the check in mlock_fixup(). However, they will be locked indirectly
if they are created after mlockall(MCL_FUTURE).

Fixes: 9651fcedf7b9 ("mm: add MAP_DROPPABLE for designating always lazily freeable mappings")
Signed-off-by: Anthony Yznaga <anthony.yznaga@oracle.com>
---
 include/linux/mm.h | 3 +++
 mm/mlock.c         | 4 ++--
 mm/vma.c           | 2 +-
 3 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 5be3d8a8f806..bb830574d112 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -574,6 +574,9 @@ enum {
 /* This mask represents all the VMA flag bits used by mlock */
 #define VM_LOCKED_MASK	(VM_LOCKED | VM_LOCKONFAULT)
 
+/* This mask prevents VMAs from being mlock'd */
+#define VM_NO_MLOCK_MASK	(VM_SPECIAL | VM_DROPPABLE)
+
 /* These flags can be updated atomically via VMA/mmap read lock. */
 #define VM_ATOMIC_SET_ALLOWED VM_MAYBE_GUARD
 
diff --git a/mm/mlock.c b/mm/mlock.c
index 2f699c3497a5..fd35c1e88c4c 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -472,9 +472,9 @@ static int mlock_fixup(struct vma_iterator *vmi, struct vm_area_struct *vma,
 	int ret = 0;
 	vm_flags_t oldflags = vma->vm_flags;
 
-	if (newflags == oldflags || (oldflags & VM_SPECIAL) ||
+	if (newflags == oldflags || (oldflags & VM_NO_MLOCK_MASK) ||
 	    is_vm_hugetlb_page(vma) || vma == get_gate_vma(current->mm) ||
-	    vma_is_dax(vma) || vma_is_secretmem(vma) || (oldflags & VM_DROPPABLE))
+	    vma_is_dax(vma) || vma_is_secretmem(vma))
 		/* don't set VM_LOCKED or VM_LOCKONFAULT and don't count */
 		goto out;
 
diff --git a/mm/vma.c b/mm/vma.c
index be64f781a3aa..1334622e4a03 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -2589,7 +2589,7 @@ static void __mmap_complete(struct mmap_state *map, struct vm_area_struct *vma)
 
 	vm_stat_account(mm, vma->vm_flags, map->pglen);
 	if (vm_flags & VM_LOCKED) {
-		if ((vm_flags & VM_SPECIAL) || vma_is_dax(vma) ||
+		if ((vm_flags & VM_NO_MLOCK_MASK) || vma_is_dax(vma) ||
 					is_vm_hugetlb_page(vma) ||
 					vma == get_gate_vma(mm))
 			vm_flags_clear(vma, VM_LOCKED_MASK);
-- 
2.47.3