Re: [PATCH v4 00/21] mm: expand mmap_prepare functionality and usage

Andrew Morton
2026-03-21T02:43:05Z

On Fri, 20 Mar 2026 22:39:26 +0000 "Lorenzo Stoakes (Oracle)" <ljs@kernel.org> wrote:

> This series expands the mmap_prepare functionality, which is intended to
> replace the deprecated f_op->mmap hook which has been the source of bugs
> and security issues for some time.

Thanks, I updated mm-unstable to this version. Here's how that altered
mm.git:

Documentation/filesystems/mmap_prepare.rst | 4
include/linux/mm.h | 3
mm/internal.h | 27 ++++-
mm/util.c | 87 +++++++++----------
mm/vma.c | 41 +-------
tools/testing/vma/include/dup.h | 44 +--------
tools/testing/vma/include/stubs.h | 3
7 files changed, 80 insertions(+), 129 deletions(-)

--- a/Documentation/filesystems/mmap_prepare.rst~b
+++ a/Documentation/filesystems/mmap_prepare.rst
@@ -123,8 +123,8 @@ When implementing mmap_prepare(), refere
as a ``VMA_xxx_BIT`` macro, e.g. ``VMA_READ_BIT``, ``VMA_WRITE_BIT`` etc.,
and use one of (where ``desc`` is a pointer to struct vm_area_desc):

-* ``vma_desc_test_flags(desc, ...)`` - Specify a comma-separated list of flags
- you wish to test for (whether _any_ are set), e.g. - ``vma_desc_test_flags(
+* ``vma_desc_test_any(desc, ...)`` - Specify a comma-separated list of flags
+ you wish to test for (whether _any_ are set), e.g. - ``vma_desc_test_any(
desc, VMA_WRITE_BIT, VMA_MAYWRITE_BIT)`` - returns ``true`` if either are set,
otherwise ``false``.
* ``vma_desc_set_flags(desc, ...)`` - Update the VMA descriptor flags to set
--- a/include/linux/mm.h~b
+++ a/include/linux/mm.h
@@ -4394,8 +4394,7 @@ static inline void mmap_action_map_kerne

int mmap_action_prepare(struct vm_area_desc *desc);
int mmap_action_complete(struct vm_area_struct *vma,
- struct mmap_action *action,
- bool rmap_lock_held);
+ struct mmap_action *action);

/* Look up the first VMA which exactly match the interval vm_start ... vm_end */
static inline struct vm_area_struct *find_exact_vma(struct mm_struct *mm,
--- a/mm/internal.h~b
+++ a/mm/internal.h
@@ -202,14 +202,6 @@ static inline void vma_close(struct vm_a
/* unmap_vmas is in mm/memory.c */
void unmap_vmas(struct mmu_gather *tlb, struct unmap_desc *unmap);

-static inline void unmap_vma_locked(struct vm_area_struct *vma)
-{
- const size_t len = vma_pages(vma) << PAGE_SHIFT;
-
- mmap_assert_write_locked(vma->vm_mm);
- do_munmap(vma->vm_mm, vma->vm_start, len, NULL);
-}
-
#ifdef CONFIG_MMU

static inline void get_anon_vma(struct anon_vma *anon_vma)
@@ -1826,6 +1818,25 @@ static inline int io_remap_pfn_range_pre
return 0;
}

+/*
+ * When we succeed an mmap action or just before we unmap a VMA on error, we
+ * need to ensure any rmap lock held is released. On unmap it's required to
+ * avoid a deadlock.
+ */
+static inline void maybe_rmap_unlock_action(struct vm_area_struct *vma,
+ struct mmap_action *action)
+{
+ struct file *file;
+
+ if (!action->hide_from_rmap_until_complete)
+ return;
+
+ VM_WARN_ON_ONCE(vma_is_anonymous(vma));
+ file = vma->vm_file;
+ i_mmap_unlock_write(file->f_mapping);
+ action->hide_from_rmap_until_complete = false;
+}
+
#ifdef CONFIG_MMU_NOTIFIER
static inline int clear_flush_young_ptes_notify(struct vm_area_struct *vma,
unsigned long addr, pte_t *ptep, unsigned int nr)
--- a/mm/util.c~b
+++ a/mm/util.c
@@ -1198,25 +1198,6 @@ void compat_set_desc_from_vma(struct vm_
}
EXPORT_SYMBOL(compat_set_desc_from_vma);

-static int __compat_vma_mapped(struct file *file, struct vm_area_struct *vma)
-{
- const struct vm_operations_struct *vm_ops = vma->vm_ops;
- void *vm_private_data = vma->vm_private_data;
- int err;
-
- if (!vm_ops || !vm_ops->mapped)
- return 0;
-
- err = vm_ops->mapped(vma->vm_start, vma->vm_end, vma->vm_pgoff, file,
- &vm_private_data);
- if (err)
- unmap_vma_locked(vma);
- else if (vm_private_data != vma->vm_private_data)
- vma->vm_private_data = vm_private_data;
-
- return err;
-}
-
/**
* __compat_vma_mmap() - Similar to compat_vma_mmap(), only it allows
* flexibility as to how the mmap_prepare callback is invoked, which is useful
@@ -1251,13 +1232,7 @@ int __compat_vma_mmap(struct vm_area_des
/* Update the VMA from the descriptor. */
compat_set_vma_from_desc(vma, desc);
/* Complete any specified mmap actions. */
- err = mmap_action_complete(vma, &desc->action,
- /*rmap_lock_held=*/false);
- if (err)
- return err;
-
- /* Invoke vm_ops->mapped callback. */
- return __compat_vma_mapped(desc->file, vma);
+ return mmap_action_complete(vma, &desc->action);
}
EXPORT_SYMBOL(__compat_vma_mmap);

@@ -1290,12 +1265,17 @@ EXPORT_SYMBOL(__compat_vma_mmap);
int compat_vma_mmap(struct file *file, struct vm_area_struct *vma)
{
struct vm_area_desc desc;
+ struct mmap_action *action;
int err;

compat_set_desc_from_vma(&desc, file, vma);
err = vfs_mmap_prepare(file, &desc);
if (err)
return err;
+ action = &desc.action;
+
+ /* being invoked from .mmmap means we don't have to enforce this. */
+ action->hide_from_rmap_until_complete = false;

return __compat_vma_mmap(&desc, vma);
}
@@ -1399,25 +1379,47 @@ again:
}
}

+static int call_vma_mapped(struct vm_area_struct *vma)
+{
+ const struct vm_operations_struct *vm_ops = vma->vm_ops;
+ void *vm_private_data = vma->vm_private_data;
+ int err;
+
+ if (!vm_ops || !vm_ops->mapped)
+ return 0;
+
+ err = vm_ops->mapped(vma->vm_start, vma->vm_end, vma->vm_pgoff,
+ vma->vm_file, &vm_private_data);
+ if (err)
+ return err;
+
+ if (vm_private_data != vma->vm_private_data)
+ vma->vm_private_data = vm_private_data;
+ return 0;
+}
+
static int mmap_action_finish(struct vm_area_struct *vma,
- struct mmap_action *action, int err,
- bool rmap_lock_held)
+ struct mmap_action *action, int err)
{
- if (rmap_lock_held)
- i_mmap_unlock_write(vma->vm_file->f_mapping);
+ size_t len;

- if (!err) {
- if (action->success_hook)
- return action->success_hook(vma);
+ if (!err)
+ err = call_vma_mapped(vma);
+ if (!err && action->success_hook)
+ err = action->success_hook(vma);
+
+ /* do_munmap() might take rmap lock, so release if held. */
+ maybe_rmap_unlock_action(vma, action);
+ if (!err)
return 0;
- }

/*
* If an error occurs, unmap the VMA altogether and return an error. We
* only clear the newly allocated VMA, since this function is only
* invoked if we do NOT merge, so we only clean up the VMA we created.
*/
- unmap_vma_locked(vma);
+ len = vma_pages(vma) << PAGE_SHIFT;
+ do_munmap(current->mm, vma->vm_start, len, NULL);
if (action->error_hook) {
/* We may want to filter the error. */
err = action->error_hook(err);
@@ -1459,16 +1461,13 @@ EXPORT_SYMBOL(mmap_action_prepare);
* mmap_action_complete - Execute VMA descriptor action.
* @vma: The VMA to perform the action upon.
* @action: The action to perform.
- * @rmap_lock_held: Is the file rmap lock held?
*
* Similar to mmap_action_prepare().
*
* Return: 0 on success, or error, at which point the VMA will be unmapped.
*/
int mmap_action_complete(struct vm_area_struct *vma,
- struct mmap_action *action,
- bool rmap_lock_held)
-
+ struct mmap_action *action)
{
int err = 0;

@@ -1489,8 +1488,7 @@ int mmap_action_complete(struct vm_area_
break;
}

- return mmap_action_finish(vma, action, err,
- rmap_lock_held);
+ return mmap_action_finish(vma, action, err);
}
EXPORT_SYMBOL(mmap_action_complete);
#else
@@ -1512,8 +1510,7 @@ int mmap_action_prepare(struct vm_area_d
EXPORT_SYMBOL(mmap_action_prepare);

int mmap_action_complete(struct vm_area_struct *vma,
- struct mmap_action *action,
- bool rmap_lock_held)
+ struct mmap_action *action)
{
int err = 0;

@@ -1523,14 +1520,14 @@ int mmap_action_complete(struct vm_area_
case MMAP_REMAP_PFN:
case MMAP_IO_REMAP_PFN:
case MMAP_SIMPLE_IO_REMAP:
- casr MMAP_MAP_KERNEL_PAGES:
+ case MMAP_MAP_KERNEL_PAGES:
WARN_ON_ONCE(1); /* nommu cannot handle this. */

err = -EINVAL;
break;
}

- return mmap_action_finish(vma, action, err, rmap_lock_held);
+ return mmap_action_finish(vma, action, err);
}
EXPORT_SYMBOL(mmap_action_complete);
#endif
--- a/mm/vma.c~b
+++ a/mm/vma.c
@@ -38,8 +38,6 @@ struct mmap_state {

/* Determine if we can check KSM flags early in mmap() logic. */
bool check_ksm_early :1;
- /* If we map new, hold the file rmap lock on mapping. */
- bool hold_file_rmap_lock :1;
/* If .mmap_prepare changed the file, we don't need to pin. */
bool file_doesnt_need_get :1;
};
@@ -2530,10 +2528,12 @@ static int __mmap_new_file_vma(struct mm
*
* @map: Mapping state.
* @vmap: Output pointer for the new VMA.
+ * @action: Any mmap_prepare action that is still to complete.
*
* Returns: Zero on success, or an error.
*/
-static int __mmap_new_vma(struct mmap_state *map, struct vm_area_struct **vmap)
+static int __mmap_new_vma(struct mmap_state *map, struct vm_area_struct **vmap,
+ struct mmap_action *action)
{
struct vma_iterator *vmi = map->vmi;
int error = 0;
@@ -2582,7 +2582,7 @@ static int __mmap_new_vma(struct mmap_st
vma_start_write(vma);
vma_iter_store_new(vmi, vma);
map->mm->map_count++;
- vma_link_file(vma, map->hold_file_rmap_lock);
+ vma_link_file(vma, action->hide_from_rmap_until_complete);

/*
* vma_merge_new_range() calls khugepaged_enter_vma() too, the below
@@ -2649,8 +2649,6 @@ static int call_action_prepare(struct mm
if (err)
return err;

- if (desc->action.hide_from_rmap_until_complete)
- map->hold_file_rmap_lock = true;
return 0;
}

@@ -2731,30 +2729,6 @@ static bool can_set_ksm_flags_early(stru
return false;
}

-static int call_mapped_hook(struct mmap_state *map,
- struct vm_area_struct *vma)
-{
- const struct vm_operations_struct *vm_ops = vma->vm_ops;
- void *vm_private_data = vma->vm_private_data;
- int err;
-
- if (!vm_ops || !vm_ops->mapped)
- return 0;
- err = vm_ops->mapped(vma->vm_start, vma->vm_end, vma->vm_pgoff,
- vma->vm_file, &vm_private_data);
- if (err) {
- if (map->hold_file_rmap_lock)
- i_mmap_unlock_write(vma->vm_file->f_mapping);
-
- unmap_vma_locked(vma);
- return err;
- }
- /* Update private data if changed. */
- if (vm_private_data != vma->vm_private_data)
- vma->vm_private_data = vm_private_data;
- return 0;
-}
-
static unsigned long __mmap_region(struct file *file, unsigned long addr,
unsigned long len, vma_flags_t vma_flags,
unsigned long pgoff, struct list_head *uf)
@@ -2794,7 +2768,7 @@ static unsigned long __mmap_region(struc

/* ...but if we can't, allocate a new VMA. */
if (!vma) {
- error = __mmap_new_vma(&map, &vma);
+ error = __mmap_new_vma(&map, &vma, &desc.action);
if (error)
goto unacct_error;
allocated_new = true;
@@ -2806,10 +2780,7 @@ static unsigned long __mmap_region(struc
__mmap_complete(&map, vma);

if (have_mmap_prepare && allocated_new) {
- error = mmap_action_complete(vma, &desc.action,
- map.hold_file_rmap_lock);
- if (!error)
- error = call_mapped_hook(&map, vma);
+ error = mmap_action_complete(vma, &desc.action);
if (error)
return error;
}
--- a/tools/testing/vma/include/dup.h~b
+++ a/tools/testing/vma/include/dup.h
@@ -1313,27 +1313,9 @@ static inline unsigned long vma_pages(co
return (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
}

-static inline void unmap_vma_locked(struct vm_area_struct *vma)
-{
- const size_t len = vma_pages(vma) << PAGE_SHIFT;
-
- mmap_assert_write_locked(vma->vm_mm);
- do_munmap(vma->vm_mm, vma->vm_start, len, NULL);
-}
-
-static inline int __compat_vma_mapped(struct file *file, struct vm_area_struct *vma)
+static inline int vfs_mmap_prepare(struct file *file, struct vm_area_desc *desc)
{
- const struct vm_operations_struct *vm_ops = vma->vm_ops;
- int err;
-
- if (!vm_ops->mapped)
- return 0;
-
- err = vm_ops->mapped(vma->vm_start, vma->vm_end, vma->vm_pgoff, file,
- &vma->vm_private_data);
- if (err)
- unmap_vma_locked(vma);
- return err;
+ return file->f_op->mmap_prepare(desc);
}

static inline int __compat_vma_mmap(struct vm_area_desc *desc,
@@ -1348,35 +1330,27 @@ static inline int __compat_vma_mmap(stru
/* Update the VMA from the descriptor. */
compat_set_vma_from_desc(vma, desc);
/* Complete any specified mmap actions. */
- err = mmap_action_complete(vma, &desc->action,
- /*rmap_lock_held=*/false);
- if (err)
- return err;
-
- /* Invoke vm_ops->mapped callback. */
- return __compat_vma_mapped(desc->file, vma);
-}
-
-static inline int vfs_mmap_prepare(struct file *file, struct vm_area_desc *desc)
-{
- return file->f_op->mmap_prepare(desc);
+ return mmap_action_complete(vma, &desc->action);
}

-static inline int compat_vma_mmap(struct file *file,
- struct vm_area_struct *vma)
+static inline int compat_vma_mmap(struct file *file, struct vm_area_struct *vma)
{
struct vm_area_desc desc;
+ struct mmap_action *action;
int err;

compat_set_desc_from_vma(&desc, file, vma);
err = vfs_mmap_prepare(file, &desc);
if (err)
return err;
+ action = &desc.action;
+
+ /* being invoked from .mmmap means we don't have to enforce this. */
+ action->hide_from_rmap_until_complete = false;

return __compat_vma_mmap(&desc, vma);
}

-
static inline void vma_iter_init(struct vma_iterator *vmi,
struct mm_struct *mm, unsigned long addr)
{
--- a/tools/testing/vma/include/stubs.h~b
+++ a/tools/testing/vma/include/stubs.h
@@ -87,8 +87,7 @@ static inline int mmap_action_prepare(st
}

static inline int mmap_action_complete(struct vm_area_struct *vma,
- struct mmap_action *action,
- bool rmap_lock_held)
+ struct mmap_action *action)
{
return 0;
}
_