[PATCH v3 00/40] mm: make VMA flag semantics explicit, eliminate VM_SPECIAL
Andrew Morton
akpm at linux-foundation.org
Thu Sep 17 14:23:29 PDT 2026
On Thu, 17 Sep 2026 17:22:09 +0100 "Lorenzo Stoakes (ARM)" <ljs at kernel.org> wrote:
> The VM_SPECIAL / VMA_SPECIAL_FLAGS mask conflates several unrelated
> properties:
>
> * Is this kernel-owned, whether MMIO, kernel-allocated pages, or ordinary
> pages a driver maps itself?
> * Can it be expanded or merged?
> * Is this a 'weird' case like mlock where migration might race and we
> 'have' to set invalid flags to notify?
> * Is it another 'weird' case where we just want to stop GUP from touching
> it?
>
> ...
>
> This series brings some order to things by both limiting what drivers can
> do with VMA flags and switching to using predicates that describe
> behaviour, not arbitrary flags.
Thanks, I've updated mm.git's mm-unstable branch to this version.
> v3:
> * Fixed up bug in patch 1 as reported by Mike - have to delay setting
> map->vma_flags until after action prepare, though map->vm_file needs to
> be set before for correct reference count management.
> * Updated 4/40 to add a symmetric vm_end check as well as vm_start in case
> of a dangerously insane driver, as per Sashiko.
> * Updated 8/40 to check if a driver did something REALLY stupid like having
> a NULL discontig_kernel_page_ops ptr, as per Sashiko.
> * Updated 15/40 to trivially synchronise userland test comments.
> * Updated 17/40 to correctly duplicate code to the userland VMA tests as
> per Sashiko.
Here's how v3 altered mm.git:
mm/internal.h | 4 ++-
mm/memory.c | 2 -
mm/vma.c | 34 +++++++++++++++++-------------
mm/vma.h | 3 +-
tools/testing/vma/include/dup.h | 24 ++++++++++++++++-----
5 files changed, 45 insertions(+), 22 deletions(-)
--- a/mm/internal.h~b
+++ a/mm/internal.h
@@ -244,6 +244,7 @@ static inline void vma_close(struct vm_a
static inline int mmap_file(struct file *file, struct vm_area_struct *vma)
{
const unsigned long prev_start = vma->vm_start;
+ const unsigned long prev_end = vma->vm_end;
const vma_flags_t prev_flags = vma->flags;
int err;
@@ -263,9 +264,10 @@ static inline int mmap_file(struct file
if (unlikely(err))
return err;
- err = mmap_hook_validate(prev_start, &prev_flags, vma);
+ err = mmap_hook_validate(prev_start, prev_end, &prev_flags, vma);
if (unlikely(err)) {
vma->vm_start = prev_start;
+ vma->vm_end = prev_end;
vma_close(vma);
}
--- a/mm/memory.c~b
+++ a/mm/memory.c
@@ -2653,7 +2653,7 @@ int map_discontig_kernel_pages_prepare(s
action->map_kernel_discontig.ops;
/* At minimum need to be able to get pages. */
- if (WARN_ON_ONCE(!ops->get))
+ if (WARN_ON_ONCE(!ops || !ops->get))
return -EINVAL;
__map_kernel_pages_prepare(desc);
--- a/mm/vma.c~b
+++ a/mm/vma.c
@@ -2797,15 +2797,15 @@ static int mmap_validate_vma_flags(const
}
/* Check to ensure a driver hasn't done something crazy. */
-static int mmap_validate(unsigned long prev_start,
- unsigned long curr_start,
+static int mmap_validate(unsigned long prev_start, unsigned long prev_end,
+ unsigned long curr_start, unsigned long curr_end,
const vma_flags_t *prev_flags,
const vma_flags_t *curr_flags)
{
bool was_maywrite, is_maywrite;
- /* Drivers cannot alter the address of the VMA. */
- if (WARN_ON_ONCE(prev_start != curr_start))
+ /* Drivers cannot alter the range of the VMA. */
+ if (WARN_ON_ONCE(prev_start != curr_start || prev_end != curr_end))
return -EINVAL;
was_maywrite = vma_flags_test(prev_flags, VMA_MAYWRITE_BIT);
@@ -2843,7 +2843,8 @@ int mmap_prepare_validate(const struct v
WARN_ON_ONCE(desc->action.type != MMAP_NOTHING))
return -EINVAL;
- return mmap_validate(prev_desc->start, desc->start,
+ return mmap_validate(prev_desc->start, prev_desc->end,
+ desc->start, desc->end,
&prev_desc->vma_flags, &desc->vma_flags);
}
@@ -2851,19 +2852,22 @@ int mmap_prepare_validate(const struct v
* mmap_hook_validate() - Ensure the driver hasn't violated invariants in
* its f_op->mmap hook.
* @prev_start: The start of the mapping prior to the mmap hook.
+ * @prev_end: The end of the mapping prior to the mmap hook.
* @prev_flags: The VMA flags set for the VMA prior to the mmap hook.
* @vma: The VMA after the hook has been applied.
*
* Returns: 0 on success, otherwise an error.
*/
-int mmap_hook_validate(unsigned long prev_start,
+int mmap_hook_validate(unsigned long prev_start, unsigned long prev_end,
const vma_flags_t *prev_flags,
const struct vm_area_struct *vma)
{
const unsigned long start = vma->vm_start;
+ const unsigned long end = vma->vm_end;
const vma_flags_t *flags = &vma->flags;
- return mmap_validate(prev_start, start, prev_flags, flags);
+ return mmap_validate(prev_start, prev_end, start, end, prev_flags,
+ flags);
}
static int call_action_prepare(struct mmap_state *map,
@@ -2900,15 +2904,9 @@ static int call_mmap_prepare(struct mmap
if (err)
return err;
- /* Update fields permitted to be changed. */
- map->pgoff = desc->pgoff;
+ /* Update first so file refcount tracked correctly. */
if (desc->vm_file != map->vm_file)
map->vm_file = desc->vm_file;
- map->vma_flags = desc->vma_flags;
- map->page_prot = desc->page_prot;
- /* User-defined fields. */
- map->vm_ops = desc->vm_ops;
- map->vm_private_data = desc->private_data;
/* It's invalid for mmap_prepare hooks to clear vm_ops. */
if (!desc->vm_ops)
@@ -2923,6 +2921,14 @@ static int call_mmap_prepare(struct mmap
if (err)
return err;
+ /* Update fields permitted to be changed. */
+ map->pgoff = desc->pgoff;
+ map->vma_flags = desc->vma_flags;
+ map->page_prot = desc->page_prot;
+ /* User-defined fields. */
+ map->vm_ops = desc->vm_ops;
+ map->vm_private_data = desc->private_data;
+
/*
* MAP_PRIVATE-/dev/zero mappings are an ancient way of getting
* anonymous mappings. Rather than allowing these mappings to be odd
--- a/mm/vma.h~b
+++ a/mm/vma.h
@@ -786,7 +786,7 @@ void vm_area_free(struct vm_area_struct
int mmap_prepare_validate(const struct vm_area_desc *prev_desc,
const struct vm_area_desc *desc);
-int mmap_hook_validate(unsigned long prev_start,
+int mmap_hook_validate(unsigned long prev_start, unsigned long prev_end,
const vma_flags_t *prev_flags,
const struct vm_area_struct *vma);
@@ -851,6 +851,7 @@ static inline int mmap_prepare_validate(
}
static inline int mmap_hook_validate(unsigned long prev_start,
+ unsigned long prev_end,
const vma_flags_t *prev_flags,
const struct vm_area_struct *vma)
{
--- a/tools/testing/vma/include/dup.h~b
+++ a/tools/testing/vma/include/dup.h
@@ -1667,22 +1667,36 @@ static inline bool vma_is_kernel_owned(c
return vma_flags_is_kernel_owned(&vma->flags);
}
+static inline bool vma_flags_is_fixed_mapping(const vma_flags_t *flags)
+{
+ /*
+ * VMA_PFNMAP_BIT should imply VMA_DONTEXPAND_BIT, but some callers set
+ * only the former.
+ */
+ return vma_flags_test_any(flags, VMA_PFNMAP_BIT, VMA_DONTEXPAND_BIT);
+}
+
+static inline bool vma_is_fixed_mapping(const struct vm_area_struct *vma)
+{
+ return vma_flags_is_fixed_mapping(&vma->flags);
+}
+
static inline bool vma_flags_can_merge(const vma_flags_t *flags)
{
/*
- * VMA merging assumes that the properties of a VMA completely describe
- * the properties of that VMA.
+ * VMA merging assumes that a VMA's flags and fields completely describe
+ * its state.
*
* However, kernel-owned mappings may have established state upon mapping
* not embodied in any attribute of the VMA.
*
- * Additionally, PFN maps encode the source PFN of the range in
- * vma->vm_pgoff, which may otherwise cause spurious merges.
+ * Additionally, private (CoW) PFN maps encode the source PFN of the
+ * range in vma->vm_pgoff, which may otherwise cause spurious merges.
*/
if (vma_flags_is_kernel_owned(flags))
return false;
/* VMA explicitly marked as being unmergeable. */
- if (vma_flags_test(flags, VMA_DONTEXPAND_BIT))
+ if (vma_flags_is_fixed_mapping(flags))
return false;
return true;
_
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