[PATCH v6 00/39] mm: make VMA flag semantics explicit, eliminate VM_SPECIAL
Andrew Morton
akpm at linux-foundation.org
Tue Oct 13 11:50:40 PDT 2026
On Tue, 13 Oct 2026 16:04:40 +0100 "Lorenzo Stoakes (ARM)" <ljs at kernel.org> wrote:
> The VM_SPECIAL / VMA_SPECIAL_FLAGS mask conflates several unrelated
> properties:
>
> * Is this managed by core mm, or by a driver mapping MMIO, kernel-allocated
> pages, or even ordinary pages 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?
>
> Driver writers have often been confused about this, and who can blame them?
>
> It also interacts badly with the eternal edgecase known as hugetlb - which
> sets VMA_DONTEXPAND_BIT but doesn't also want to be treated like a
> 'special' flag.
>
> Another issue is that we cannot make sensible assumptions about flag
> use. It's not possible to assume VMA_IO_BIT means iommu because drivers
> abuse it and mlock abuses it.
>
> Special is also an overloaded term in mm. VDSO and VVAR mappings are also
> called 'special' but they're special in a... special way.
>
> Sometimes things are called special that are a subset of
> VMA_SPECIAL_FLAGS (VMA_PFNMAP_BIT and VMA_MIXEDMAP_BIT for instance when it
> comes to zapping or vm_normal_folio()).
>
> There's a specific kind of special for THP too, which considers
> PFN map, mixed map 'special' but DAX not.
>
> It's all rather a mess.
>
> 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-unstable to this version.
> v6:
> * Applied tags (thanks David!)
> * Added 7/39 to split out the vm_insert_page() / vm_insert_pages()
> mmap lock assertions and VMA API changes out into their own patch,
> as per David.
> * Updated 8/39 (was 7/38) to collapse the discontig single page case to
> a single one and to rename things, as per David.
> * Updated 9/39 (was 8/38) to update the discontig docs to reflect the
> change.
> * Updated 23/39 (was 22/38) to put expression on single line, as per David.
> * Updated 25/39 (was 24/38) to mention the open-coding in the commit
> message (fixed in next commit), as per discussion with David.
> * Updated 26/39 (was 25/38) to clarify the commit message around the
> removal of the VMA_IO_BIT check in vma_flags_is_mm_managed(), as per
> David.
> * Updated 30/39 (was 30/38) to no longer exclude droppable mappings from
> vma_flags_is_mm_backed(), as it doesn't quite fit into the broader
> definition, as per David.
> * Updated 30/39 (was 30/38) to express vma_flags_can_merge() in terms of
> vma_flags_is_mm_backed() (with an explicit hugetlb exclusion), and to
> no longer use vma_can_merge() in __thp_vma_allowable_orders().
> * Updated 31/39 (was 29/38) to use vma_is_mm_backed() (with hugetlb
> exclusion) rather than vma_can_merge(), as guard regions have nothing to
> do with mergeability.
> * Updated 32/39 (was 31/38) to retain an explicit droppable check in
> vma_can_userfault(), as per David.
> * Updated 33/39 (was 32/38) to use vma_is_mm_backed() with an explicit
> hugetlb exclusion, rather than vma_can_merge() which makes no sense here.
> * Updated 34/39 (was 33/38) to retain explicit droppable checks in
> vma_supports_mlock() and ksm_compatible(), as per David.
> * Updated 34/39 (was 33/38) to use vma_is_mm_backed() in should_skip_vma(),
> based on discussion with David.
> * Updated 35/39 (was 34/38) to use vma_is_mm_backed() in
> lru_gen_look_around(), as per David.
> * Updated 36/39 (was 35/38) to use vma_is_mm_backed() with an explicit
> hugetlb exclusion in migrate_vma_setup(), as per David.
Here's how v6 altered mm.git.
Documentation/filesystems/mmap_prepare.rst | 2
include/linux/mm.h | 120 +++++++++----------
include/linux/mm_types.h | 6
mm/huge_memory.c | 7 -
mm/internal.h | 3
mm/ksm.c | 3
mm/madvise.c | 11 +
mm/memory.c | 16 +-
mm/migrate_device.c | 4
mm/userfaultfd.c | 3
mm/vma.c | 4
mm/vmscan.c | 7 -
tools/testing/vma/include/dup.h | 44 +++++-
13 files changed, 128 insertions(+), 102 deletions(-)
--- a/Documentation/filesystems/mmap_prepare.rst~b
+++ a/Documentation/filesystems/mmap_prepare.rst
@@ -240,7 +240,7 @@ In the ``get`` hook, the user must choos
compound pages (if the compound page is bigger than the remaining pages in the
VMA, then only those pages that fit will be mapped). For a compound page, the
head page must be passed.
-* ``discontig_kernel_map_page_range()`` - Map an array of pages of a specified
+* ``discontig_kernel_map_pages()`` - Map an array of pages of a specified
size. As with a compound page, if more pages are specified than remain in
the VMA then only those that fit will be mapped.
--- a/include/linux/mm.h~b
+++ a/include/linux/mm.h
@@ -1695,57 +1695,22 @@ static inline bool vma_is_fixed_mapping(
}
/**
- * vma_flags_can_merge() - Do the specified VMA flags permit the VMA to be
- * merged with another?
- * @flags: The VMA flags to test.
- * Returns: true if the flags permit merging, false otherwise.
- */
-static inline bool vma_flags_can_merge(const vma_flags_t *flags)
-{
- /*
- * VMA merging assumes that a VMA's flags and fields completely describe
- * its state.
- *
- * However, mappings which are not mm-managed may have established state
- * upon mapping not embodied in any attribute of the VMA.
- *
- * 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_mm_managed(flags))
- return false;
- /* VMA explicitly marked as being unmergeable. */
- if (vma_flags_is_fixed_mapping(flags))
- return false;
-
- return true;
-}
-
-/**
- * vma_can_merge() - Do @vma's flags permit it to be merged with another VMA?
- * @vma: The VMA to test.
- * Returns: true if the flags permit merging, otherwise false.
- */
-static inline bool vma_can_merge(const struct vm_area_struct *vma)
-{
- return vma_flags_can_merge(&vma->flags);
-}
-
-/**
* vma_flags_is_mm_backed() - Do the specified VMA flags imply the mapping is
* backed by core mm?
* @flags: The VMA flags to test.
*
* An mm-backed mapping is one managed by core mm (see
- * vma_flags_is_mm_managed()) for which two further things are true:
+ * vma_flags_is_mm_managed()) whose contents are established by core mm's own
+ * fault path rather than a custom ->fault handler.
*
- * 1. Everything mapped was placed there by core mm's fault path (no custom
- * ->fault).
- * 2. What is mapped stays there until it is zapped or unmapped by the user.
- *
- * These mappings are the only ones which can be sensibly locked, merged, dumped
- * or be subject to uffd faulting, as all of these assume core mm and core mm
- * alone manages these, and that the mappings will remain there.
+ * Mappings not managed by core mm are populated by their owner, and fixed
+ * mappings may be mm-managed but have their contents established by a custom
+ * ->fault handler, so neither are mm-backed. hugetlb mappings are fixed, but
+ * core mm owns them entirely, so they are.
+ *
+ * These are the only mappings which can sensibly be merged, locked, dumped or
+ * be subject to uffd faulting, as all of these assume core mm and core mm
+ * alone manages their contents.
*
* Returns: true if the flags imply an mm-backed mapping, otherwise false.
*/
@@ -1761,11 +1726,7 @@ static inline bool vma_flags_is_mm_backe
* by a custom ->fault handler, so core mm cannot assume ordinary fault
* semantics over them.
*/
- if (!vma_flags_is_mm_managed(flags) ||
- vma_flags_is_fixed_mapping(flags))
- return false;
- /* Core mm may discard droppable memory at any time. */
- return !vma_flags_test_single_mask(flags, VMA_DROPPABLE);
+ return vma_flags_is_mm_managed(flags) && !vma_flags_is_fixed_mapping(flags);
}
/**
@@ -1782,6 +1743,43 @@ static inline bool vma_is_mm_backed(cons
}
/**
+ * vma_flags_can_merge() - Do the specified VMA flags permit the VMA to be
+ * merged with another?
+ * @flags: The VMA flags to test.
+ * Returns: true if the flags permit merging, false otherwise.
+ */
+static inline bool vma_flags_can_merge(const vma_flags_t *flags)
+{
+ /*
+ * VMA merging assumes that a VMA's flags and fields completely describe
+ * its state, which is only the case for mm-backed mappings.
+ *
+ * Mappings which are not mm-managed may have established state upon
+ * mapping not embodied in any attribute of the VMA.
+ *
+ * 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_mm_backed(flags))
+ return false;
+ /*
+ * hugetlb mappings contain per-VMA state, so their flags and fields do
+ * not completely describe them.
+ */
+ return !vma_flags_is_hugetlb(flags);
+}
+
+/**
+ * vma_can_merge() - Do @vma's flags permit it to be merged with another VMA?
+ * @vma: The VMA to test.
+ * Returns: true if the flags permit merging, otherwise false.
+ */
+static inline bool vma_can_merge(const struct vm_area_struct *vma)
+{
+ return vma_flags_can_merge(&vma->flags);
+}
+
+/**
* vma_kernel_pagesize - Default page size granularity for this VMA.
* @vma: The user mapping.
*
@@ -4824,28 +4822,26 @@ discontig_kernel_map_abort(struct discon
static inline void
discontig_kernel_map_page(struct discontig_kernel_page_state *state,
- struct page *page)
+ struct page *page)
{
struct folio *folio = page_folio(page);
+ unsigned long nr_pages = 1;
if (folio_test_large(folio)) {
VM_WARN_ON_ONCE(page != folio_page(folio, 0));
- state->action = DISCONTIG_KERNEL_PAGE_MAP_COMPOUND_PAGE;
- state->__folio = folio;
- state->__nr_pages = min(state->nr_pages_remain,
- folio_nr_pages(folio));
- } else {
- state->action = DISCONTIG_KERNEL_PAGE_MAP_PAGE;
- state->__page = page;
- state->__nr_pages = 1;
+ nr_pages = folio_nr_pages(folio);
}
+
+ state->action = DISCONTIG_KERNEL_PAGE_MAP_CONTIG_PAGES;
+ state->__page = page;
+ state->__nr_pages = min(nr_pages, state->nr_pages_remain);
}
static inline void
-discontig_kernel_map_page_range(struct discontig_kernel_page_state *state,
- struct page **page_arr, unsigned long nr_pages)
+discontig_kernel_map_pages(struct discontig_kernel_page_state *state,
+ struct page **page_arr, unsigned long nr_pages)
{
- state->action = DISCONTIG_KERNEL_PAGE_MAP_PAGE_RANGE;
+ state->action = DISCONTIG_KERNEL_PAGE_MAP_PAGES;
state->__page_arr = page_arr;
state->__nr_pages = min(nr_pages, state->nr_pages_remain);
}
--- a/include/linux/mm_types.h~b
+++ a/include/linux/mm_types.h
@@ -825,9 +825,8 @@ enum mmap_action_type {
enum discontig_kernel_page_action {
DISCONTIG_KERNEL_PAGE_ABORT,
- DISCONTIG_KERNEL_PAGE_MAP_PAGE,
- DISCONTIG_KERNEL_PAGE_MAP_COMPOUND_PAGE,
- DISCONTIG_KERNEL_PAGE_MAP_PAGE_RANGE,
+ DISCONTIG_KERNEL_PAGE_MAP_PAGES, /* Array of individual pages. */
+ DISCONTIG_KERNEL_PAGE_MAP_CONTIG_PAGES, /* 1..N contiguous pages. */
};
struct discontig_kernel_page_state {
@@ -847,7 +846,6 @@ struct discontig_kernel_page_state {
enum discontig_kernel_page_action action;
union {
struct page *__page;
- struct folio *__folio;
struct page **__page_arr;
};
unsigned long __nr_pages;
--- a/mm/huge_memory.c~b
+++ a/mm/huge_memory.c
@@ -207,11 +207,10 @@ unsigned long __thp_vma_allowable_orders
* khugepaged moves data from VMAs once collapsed, after they have been
* faulted in, relying on refaulting for file-backed memory.
*
- * Mappings not managed by core mm cannot be reliably reconstructed from
- * page faults, and fixed mappings (including hugetlb) may well be
- * mm-managed - precisely the mappings which cannot be merged.
+ * Only mm-backed mappings can be reliably reconstructed from page
+ * faults, and hugetlb mappings cannot be collapsed.
*/
- if (!in_pf && !smaps && !vma_can_merge(vma))
+ if (!in_pf && !smaps && (!vma_is_mm_backed(vma) || vma_is_hugetlb(vma)))
return 0;
/*
--- a/mm/internal.h~b
+++ a/mm/internal.h
@@ -1114,6 +1114,9 @@ static inline bool vma_supports_mlock(co
{
if (!vma_is_mm_backed(vma))
return false;
+ /* Core mm may discard droppable memory at any time. */
+ if (vma_test_single_mask(vma, VMA_DROPPABLE))
+ return false;
if (vma_is_dax(vma) || vma_is_hugetlb(vma))
return false;
return vma != get_gate_vma(current->mm);
--- a/mm/ksm.c~b
+++ a/mm/ksm.c
@@ -749,6 +749,9 @@ static bool ksm_compatible(const struct
return false;
if (!vma_flags_is_mm_backed(&vma_flags))
return false;
+ /* Core mm may discard droppable memory at any time. */
+ if (vma_flags_test_single_mask(&vma_flags, VMA_DROPPABLE))
+ return false;
if (file_is_dax(file))
return false;
#ifdef VM_SAO
--- a/mm/madvise.c~b
+++ a/mm/madvise.c
@@ -1069,10 +1069,10 @@ static bool is_valid_guard_vma(const str
return false;
/*
* Guard regions require a VMA whose page tables are managed solely by
- * the core, which is also what merging requires, so disallow any flags
- * that would prevent a merge.
+ * core mm, so the VMA must be mm-backed (excluding hugetlb, in which
+ * guard markers cannot be installed).
*/
- if (!vma_can_merge(vma))
+ if (!vma_is_mm_backed(vma) || vma_is_hugetlb(vma))
return false;
return true;
@@ -1402,7 +1402,7 @@ static int madvise_vma_behavior(struct m
new_flags |= VM_DONTCOPY;
break;
case MADV_DOFORK:
- if (!vma_can_merge(vma))
+ if (!vma_is_mm_backed(vma) || vma_is_hugetlb(vma))
return -EINVAL;
new_flags &= ~VM_DONTCOPY;
break;
@@ -1424,6 +1424,9 @@ static int madvise_vma_behavior(struct m
/* Only mm-backed memory can be meaningfully dumped. */
if (!vma_is_mm_backed(vma))
return -EINVAL;
+ /* Core mm may discard droppable memory at any time. */
+ if (new_flags & VM_DROPPABLE)
+ return -EINVAL;
new_flags &= ~VM_DONTDUMP;
break;
case MADV_MERGEABLE:
--- a/mm/memory.c~b
+++ a/mm/memory.c
@@ -2674,26 +2674,24 @@ int map_discontig_kernel_pages_prepare(s
}
static int apply_discontig_action(struct vm_area_struct *vma,
- struct discontig_kernel_page_state *state)
+ struct discontig_kernel_page_state *state)
{
unsigned long nr_pages = state->__nr_pages;
unsigned long addr = state->addr;
unsigned long i;
- if (state->action == DISCONTIG_KERNEL_PAGE_MAP_PAGE)
- return insert_page(vma, addr, state->__page,
- vma->vm_page_prot, /*mkwrite=*/false);
- if (state->action == DISCONTIG_KERNEL_PAGE_MAP_PAGE_RANGE)
+ if (state->action == DISCONTIG_KERNEL_PAGE_MAP_PAGES)
return insert_pages(vma, addr, state->__page_arr,
- &nr_pages, vma->vm_page_prot);
+ &nr_pages, vma->vm_page_prot);
- /* Compound folio - have to iterate through each page. */
+ VM_WARN_ON_ONCE(state->action != DISCONTIG_KERNEL_PAGE_MAP_CONTIG_PAGES);
+ /* Contiguous pages - have to iterate through each one. */
for (i = 0; i < nr_pages; i++, addr += PAGE_SIZE) {
- struct page *page = folio_page(state->__folio, i);
+ struct page *page = state->__page + i;
int err;
err = insert_page(vma, addr, page, vma->vm_page_prot,
- /*mkwrite=*/false);
+ /*mkwrite=*/false);
if (err)
return err;
}
--- a/mm/migrate_device.c~b
+++ a/mm/migrate_device.c
@@ -746,8 +746,8 @@ int migrate_vma_setup(struct migrate_vma
args->end &= PAGE_MASK;
if (!vma)
return -EINVAL;
- if (!vma_is_mm_managed(vma) || vma_is_fixed_mapping(vma) ||
- vma_is_dax(vma))
+ /* The migrate_vma page walk has no hugetlb support. */
+ if (!vma_is_mm_backed(vma) || vma_is_hugetlb(vma) || vma_is_dax(vma))
return -EINVAL;
if (nr_pages <= 0)
return -EINVAL;
--- a/mm/userfaultfd.c~b
+++ a/mm/userfaultfd.c
@@ -2156,6 +2156,9 @@ static bool vma_can_userfault(struct vm_
/* Only mm-backed memory can have its faults handled by userspace. */
if (!vma_is_mm_backed(vma))
return false;
+ /* Core mm may discard droppable memory at any time. */
+ if (vma_test_single_mask(vma, VMA_DROPPABLE))
+ return false;
/* The shadow stack should not be written to by userspace. */
if (vma_test_single_mask(vma, VMA_SHADOW_STACK))
return false;
--- a/mm/vma.c~b
+++ a/mm/vma.c
@@ -2654,9 +2654,7 @@ static int __mmap_new_file_vma(struct mm
* VMA_LOCKED_BIT and/or VMA_LOCKONFAULT_BIT flags.
*/
if (vma_supports_mlock(vma)) {
- const vma_flags_t mask =
- vma_flags_and_mask(&map->vma_flags,
- VMA_LOCKED_MASK);
+ const vma_flags_t mask = vma_flags_and_mask(&map->vma_flags, VMA_LOCKED_MASK);
vma_set_flags_mask(vma, mask);
}
--- a/mm/vmscan.c~b
+++ a/mm/vmscan.c
@@ -3521,8 +3521,7 @@ static int should_skip_vma(unsigned long
if (!vma_has_recency(vma))
return true;
- if (vma_test(vma, VMA_LOCKED_BIT) || !vma_is_mm_managed(vma) ||
- vma_is_fixed_mapping(vma))
+ if (vma_test(vma, VMA_LOCKED_BIT) || !vma_is_mm_backed(vma))
return true;
if (vma == get_gate_vma(vma->vm_mm))
@@ -4462,8 +4461,8 @@ bool lru_gen_look_around(struct page_vma
if (spin_is_contended(pvmw->ptl))
return true;
- /* exclude fixed and non-mm-managed VMAs containing anon pages from COW */
- if (!vma_is_mm_managed(vma) || vma_is_fixed_mapping(vma))
+ /* exclude non-mm-backed VMAs containing anon pages from COW */
+ if (!vma_is_mm_backed(vma))
return true;
/* avoid taking the LRU lock under the PTL when possible */
--- a/tools/testing/vma/include/dup.h~b
+++ a/tools/testing/vma/include/dup.h
@@ -1681,23 +1681,49 @@ static inline bool vma_is_fixed_mapping(
return vma_flags_is_fixed_mapping(&vma->flags);
}
+static inline bool vma_flags_is_hugetlb(const vma_flags_t *flags)
+{
+ return IS_ENABLED(CONFIG_HUGETLB_PAGE) &&
+ vma_flags_test(flags, VMA_HUGETLB_BIT);
+}
+
+static inline bool vma_flags_is_mm_backed(const vma_flags_t *flags)
+{
+ /* hugetlb is a fixed mapping, but core mm owns it entirely. */
+ if (vma_flags_is_hugetlb(flags))
+ return true;
+ /*
+ * Mappings not managed by core mm are populated by their owner.
+ *
+ * Fixed mappings may be mm-managed, but their contents are established
+ * by a custom ->fault handler, so core mm cannot assume ordinary fault
+ * semantics over them.
+ */
+ return vma_flags_is_mm_managed(flags) && !vma_flags_is_fixed_mapping(flags);
+}
+
+static inline bool vma_is_mm_backed(const struct vm_area_struct *vma)
+{
+ return vma_flags_is_mm_backed(&vma->flags);
+}
+
static inline bool vma_flags_can_merge(const vma_flags_t *flags)
{
/*
* VMA merging assumes that a VMA's flags and fields completely describe
- * its state.
+ * its state, which is only the case for mm-backed mappings.
*
- * However, mappings which are not mm-managed may have established state
- * upon mapping not embodied in any attribute of the VMA.
+ * Mappings which are not mm-managed may have established state upon
+ * mapping not embodied in any attribute of the VMA.
*
* 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_mm_managed(flags))
+ if (!vma_flags_is_mm_backed(flags))
return false;
- /* VMA explicitly marked as being unmergeable. */
- if (vma_flags_is_fixed_mapping(flags))
- return false;
-
- return true;
+ /*
+ * hugetlb mappings contain per-VMA state, so their flags and fields do
+ * not completely describe them.
+ */
+ return !vma_flags_is_hugetlb(flags);
}
_
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