[PATCH v4 05/21] mm: switch the rmap lock held option off in compat layer

Lorenzo Stoakes (Oracle)
2026-03-20T22:40:20Z

In the mmap_prepare compatibility layer, we don't need to hold the rmap
lock, as we are being called from an .mmap handler.

The .mmap_prepare hook, when invoked in the VMA logic, is called prior to
the VMA being instantiated, but the completion hook is called after the VMA
is linked into the maple tree, meaning rmap walkers can reach it.

The mmap hook does not link the VMA into the tree, so this cannot happen.

Therefore it's safe to simply disable this in the mmap_prepare
compatibility layer.

Also update VMA tests code to reflect current compatibility layer state.

Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
---
mm/util.c | 6 ++++-
tools/testing/vma/include/dup.h | 42 +++++++++++++++++----------------
2 files changed, 27 insertions(+), 21 deletions(-)

diff --git a/mm/util.c b/mm/util.c
index a2cfa0d77c35..182f0f5cc400 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -1204,6 +1204,7 @@ int compat_vma_mmap(struct file *file, struct vm_area_struct *vma)

.action.type = MMAP_NOTHING, /* Default */
};
+ struct mmap_action *action = &desc.action;
int err;

err = vfs_mmap_prepare(file, &desc);
@@ -1214,8 +1215,11 @@ int compat_vma_mmap(struct file *file, struct vm_area_struct *vma)
if (err)
return err;

+ /* being invoked from .mmmap means we don't have to enforce this. */
+ action->hide_from_rmap_until_complete = false;
+
set_vma_from_desc(vma, &desc);
- err = mmap_action_complete(vma, &desc.action);
+ err = mmap_action_complete(vma, action);
if (err) {
const size_t len = vma_pages(vma) << PAGE_SHIFT;

diff --git a/tools/testing/vma/include/dup.h b/tools/testing/vma/include/dup.h
index 26c6c3255a94..c62d3998e922 100644
--- a/tools/testing/vma/include/dup.h
+++ b/tools/testing/vma/include/dup.h
@@ -1256,8 +1256,17 @@ static inline void vma_set_anonymous(struct vm_area_struct *vma)
static inline void set_vma_from_desc(struct vm_area_struct *vma,
struct vm_area_desc *desc);

-static inline int __compat_vma_mmap(const struct file_operations *f_op,
- struct file *file, struct vm_area_struct *vma)
+static inline int vfs_mmap_prepare(struct file *file, struct vm_area_desc *desc)
+{
+ return file->f_op->mmap_prepare(desc);
+}
+
+static inline unsigned long vma_pages(struct vm_area_struct *vma)
+{
+ return (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
+}
+
+static inline int compat_vma_mmap(struct file *file, struct vm_area_struct *vma)
{
struct vm_area_desc desc = {
.mm = vma->vm_mm,
@@ -1272,9 +1281,10 @@ static inline int __compat_vma_mmap(const struct file_operations *f_op,

.action.type = MMAP_NOTHING, /* Default */
};
+ struct mmap_action *action = &desc.action;
int err;

- err = f_op->mmap_prepare(&desc);
+ err = vfs_mmap_prepare(file, &desc);
if (err)
return err;

@@ -1282,28 +1292,25 @@ static inline int __compat_vma_mmap(const struct file_operations *f_op,
if (err)
return err;

+ /* being invoked from .mmmap means we don't have to enforce this. */
+ action->hide_from_rmap_until_complete = false;
+
set_vma_from_desc(vma, &desc);
- return mmap_action_complete(vma, &desc.action);
-}
+ err = mmap_action_complete(vma, action);
+ if (err) {
+ const size_t len = vma_pages(vma) << PAGE_SHIFT;

-static inline int compat_vma_mmap(struct file *file,
- struct vm_area_struct *vma)
-{
- return __compat_vma_mmap(file->f_op, file, vma);
+ do_munmap(current->mm, vma->vm_start, len, NULL);
+ }
+ return err;
}

-
static inline void vma_iter_init(struct vma_iterator *vmi,
struct mm_struct *mm, unsigned long addr)
{
mas_init(&vmi->mas, &mm->mm_mt, addr);
}

-static inline unsigned long vma_pages(struct vm_area_struct *vma)
-{
- return (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
-}
-
static inline void mmap_assert_locked(struct mm_struct *);
static inline struct vm_area_struct *find_vma_intersection(struct mm_struct *mm,
unsigned long start_addr,
@@ -1473,11 +1480,6 @@ static inline int vfs_mmap(struct file *file, struct vm_area_struct *vma)
return file->f_op->mmap(file, vma);
}

-static inline int vfs_mmap_prepare(struct file *file, struct vm_area_desc *desc)
-{
- return file->f_op->mmap_prepare(desc);
-}
-
static inline void vma_set_file(struct vm_area_struct *vma, struct file *file)
{
/* Changing an anonymous vma with this is illegal */
--
2.53.0