Re: [PATCH 2/3] mm: implement sticky, copy on fork VMA flags

[Suren Baghdasaryan <surenb@google.com>]

On Wed, Oct 29, 2025 at 9:51 AM Lorenzo Stoakes
<lorenzo.stoakes@oracle.com> wrote:
>
> It's useful to be able to force a VMA to be copied on fork outside of the
> parameters specified by vma_needs_copy(), which otherwise only copies page
> tables if:
>
> * The destination VMA has VM_UFFD_WP set
> * The mapping is a PFN or mixed map
> * The mapping is anonymous and forked in (i.e. vma->anon_vma is non-NULL)
>
> Setting this flag implies that the page tables mapping the VMA are such
> that simply re-faulting the VMA will not re-establish them in identical
> form.
>
> We introduce VM_COPY_ON_FORK to clearly identify which flags require this
> behaviour, which currently is only VM_MAYBE_GUARD.
>
> Any VMA flags which require this behaviour are inherently 'sticky', that
> is, should we merge two VMAs together, this implies that the newly merged
> VMA maps a range that requires page table copying on fork.
>
> In order to implement this we must both introduce the concept of a 'sticky'
> VMA flag and adjust the VMA merge logic accordingly, and also have VMA
> merge still successfully succeed should one VMA have the flag set and
> another not.

"successfully succeed" sounds weird. Just "succeed"?

>
> Note that we update the VMA expand logic to handle new VMA merging, as this
> function is the one ultimately called by all instances of merging of new
> VMAs.
>
> This patch implements this, establishing VM_STICKY to contain all such
> flags and VM_IGNORE_MERGE for those flags which should be ignored when
> comparing adjacent VMA's flags for the purposes of merging.
>
> As part of this change we place VM_SOFTDIRTY in VM_IGNORE_MERGE as it
> already had this behaviour, alongside VM_STICKY as sticky flags by
> implication must not disallow merge.
>
> We update the VMA userland tests to account for the changes and,
> furthermore, in order to assert that the functionality is workingly

s/workingly/working

> correctly, update the new VMA and existing VMA merging logic to consider
> every permutation of the flag being set/not set in all VMAs being
> considered for merge.
>
> As a result of this change, VMAs with guard ranges will now not have their
> merge behaviour impacted by doing so and can be freely merged with other
> VMAs without VM_MAYBE_GUARD set.
>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> ---
>  include/linux/mm.h               | 32 ++++++++++++
>  mm/memory.c                      |  3 +-
>  mm/vma.c                         | 22 ++++----
>  tools/testing/vma/vma.c          | 89 ++++++++++++++++++++++++++++----
>  tools/testing/vma/vma_internal.h | 32 ++++++++++++
>  5 files changed, 156 insertions(+), 22 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index f963afa1b9de..a8811ba57150 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -522,6 +522,38 @@ extern unsigned int kobjsize(const void *objp);
>  #endif
>  #define VM_FLAGS_CLEAR (ARCH_VM_PKEY_FLAGS | VM_ARCH_CLEAR)
>
> +/* Flags which should result in page tables being copied on fork. */
> +#define VM_COPY_ON_FORK VM_MAYBE_GUARD
> +
> +/*
> + * Flags which should be 'sticky' on merge - that is, flags which, when one VMA
> + * possesses it but the other does not, the merged VMA should nonetheless have
> + * applied to it:
> + *
> + * VM_COPY_ON_FORK - These flags indicates that a VMA maps a range that contains
> + *                   metadata which should be unconditionally propagated upon
> + *                   fork. When merging two VMAs, we encapsulate this range in
> + *                   the merged VMA, so the flag should be 'sticky' as a result.

It's probably worth noting that after a split, we do not remove
"sticky" flags even if the VMA acquired them as a result of a previous
merge.

> + */
> +#define VM_STICKY VM_COPY_ON_FORK
> +
> +/*
> + * VMA flags we ignore for the purposes of merge, i.e. one VMA possessing one
> + * of these flags and the other not does not preclude a merge.
> + *
> + * VM_SOFTDIRTY - Should not prevent from VMA merging, if we match the flags but
> + *                dirty bit -- the caller should mark merged VMA as dirty. If
> + *                dirty bit won't be excluded from comparison, we increase
> + *                pressure on the memory system forcing the kernel to generate
> + *                new VMAs when old one could be extended instead.
> + *
> + *    VM_STICKY - If one VMA has flags which most be 'sticky', that is ones

s/most/must ?

> + *                which should propagate to all VMAs, but the other does not,
> + *                the merge should still proceed with the merge logic applying
> + *                sticky flags to the final VMA.
> + */
> +#define VM_IGNORE_MERGE (VM_SOFTDIRTY | VM_STICKY)
> +
>  /*
>   * mapping from the currently active vm_flags protection bits (the
>   * low four bits) to a page protection mask..
> diff --git a/mm/memory.c b/mm/memory.c
> index a2c79ee43d68..9528133e5147 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1478,8 +1478,7 @@ vma_needs_copy(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)
>         if (src_vma->anon_vma)
>                 return true;
>
> -       /* Guard regions have momdified page tables that require copying. */
> -       if (src_vma->vm_flags & VM_MAYBE_GUARD)
> +       if (src_vma->vm_flags & VM_COPY_ON_FORK)
>                 return true;
>
>         /*
> diff --git a/mm/vma.c b/mm/vma.c
> index 919d1fc63a52..50a6909c4be3 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -89,15 +89,7 @@ static inline bool is_mergeable_vma(struct vma_merge_struct *vmg, bool merge_nex
>
>         if (!mpol_equal(vmg->policy, vma_policy(vma)))
>                 return false;
> -       /*
> -        * VM_SOFTDIRTY should not prevent from VMA merging, if we
> -        * match the flags but dirty bit -- the caller should mark
> -        * merged VMA as dirty. If dirty bit won't be excluded from
> -        * comparison, we increase pressure on the memory system forcing
> -        * the kernel to generate new VMAs when old one could be
> -        * extended instead.
> -        */
> -       if ((vma->vm_flags ^ vmg->vm_flags) & ~VM_SOFTDIRTY)
> +       if ((vma->vm_flags ^ vmg->vm_flags) & ~VM_IGNORE_MERGE)
>                 return false;
>         if (vma->vm_file != vmg->file)
>                 return false;
> @@ -809,6 +801,7 @@ static bool can_merge_remove_vma(struct vm_area_struct *vma)
>  static __must_check struct vm_area_struct *vma_merge_existing_range(
>                 struct vma_merge_struct *vmg)
>  {
> +       vm_flags_t sticky_flags = vmg->vm_flags & VM_STICKY;
>         struct vm_area_struct *middle = vmg->middle;
>         struct vm_area_struct *prev = vmg->prev;
>         struct vm_area_struct *next;
> @@ -901,11 +894,13 @@ static __must_check struct vm_area_struct *vma_merge_existing_range(
>         if (merge_right) {
>                 vma_start_write(next);
>                 vmg->target = next;
> +               sticky_flags |= (next->vm_flags & VM_STICKY);
>         }
>
>         if (merge_left) {
>                 vma_start_write(prev);
>                 vmg->target = prev;
> +               sticky_flags |= (prev->vm_flags & VM_STICKY);
>         }
>
>         if (merge_both) {
> @@ -975,6 +970,7 @@ static __must_check struct vm_area_struct *vma_merge_existing_range(
>         if (err || commit_merge(vmg))
>                 goto abort;
>
> +       vm_flags_set(vmg->target, sticky_flags);
>         khugepaged_enter_vma(vmg->target, vmg->vm_flags);
>         vmg->state = VMA_MERGE_SUCCESS;
>         return vmg->target;
> @@ -1125,6 +1121,10 @@ int vma_expand(struct vma_merge_struct *vmg)
>         bool remove_next = false;
>         struct vm_area_struct *target = vmg->target;
>         struct vm_area_struct *next = vmg->next;
> +       vm_flags_t sticky_flags;
> +
> +       sticky_flags = vmg->vm_flags & VM_STICKY;
> +       sticky_flags |= target->vm_flags & VM_STICKY;
>
>         VM_WARN_ON_VMG(!target, vmg);
>
> @@ -1134,6 +1134,7 @@ int vma_expand(struct vma_merge_struct *vmg)
>         if (next && (target != next) && (vmg->end == next->vm_end)) {
>                 int ret;
>
> +               sticky_flags |= next->vm_flags & VM_STICKY;
>                 remove_next = true;
>                 /* This should already have been checked by this point. */
>                 VM_WARN_ON_VMG(!can_merge_remove_vma(next), vmg);
> @@ -1160,6 +1161,7 @@ int vma_expand(struct vma_merge_struct *vmg)
>         if (commit_merge(vmg))
>                 goto nomem;
>
> +       vm_flags_set(target, sticky_flags);
>         return 0;
>
>  nomem:
> @@ -1903,7 +1905,7 @@ static int anon_vma_compatible(struct vm_area_struct *a, struct vm_area_struct *
>         return a->vm_end == b->vm_start &&
>                 mpol_equal(vma_policy(a), vma_policy(b)) &&
>                 a->vm_file == b->vm_file &&
> -               !((a->vm_flags ^ b->vm_flags) & ~(VM_ACCESS_FLAGS | VM_SOFTDIRTY)) &&
> +               !((a->vm_flags ^ b->vm_flags) & ~(VM_ACCESS_FLAGS | VM_IGNORE_MERGE)) &&
>                 b->vm_pgoff == a->vm_pgoff + ((b->vm_start - a->vm_start) >> PAGE_SHIFT);
>  }
>
> diff --git a/tools/testing/vma/vma.c b/tools/testing/vma/vma.c
> index 656e1c75b711..ee9d3547c421 100644
> --- a/tools/testing/vma/vma.c
> +++ b/tools/testing/vma/vma.c

I prefer tests in a separate patch, but that might just be me. Feel
free to ignore.

> @@ -48,6 +48,8 @@ static struct anon_vma dummy_anon_vma;
>  #define ASSERT_EQ(_val1, _val2) ASSERT_TRUE((_val1) == (_val2))
>  #define ASSERT_NE(_val1, _val2) ASSERT_TRUE((_val1) != (_val2))
>
> +#define IS_SET(_val, _flags) ((_val & _flags) == _flags)
> +
>  static struct task_struct __current;
>
>  struct task_struct *get_current(void)
> @@ -441,7 +443,7 @@ static bool test_simple_shrink(void)
>         return true;
>  }
>
> -static bool test_merge_new(void)
> +static bool __test_merge_new(bool is_sticky, bool a_is_sticky, bool b_is_sticky, bool c_is_sticky)
>  {
>         vm_flags_t vm_flags = VM_READ | VM_WRITE | VM_MAYREAD | VM_MAYWRITE;
>         struct mm_struct mm = {};
> @@ -469,23 +471,32 @@ static bool test_merge_new(void)
>         struct vm_area_struct *vma, *vma_a, *vma_b, *vma_c, *vma_d;
>         bool merged;
>
> +       if (is_sticky)
> +               vm_flags |= VM_STICKY;
> +
>         /*
>          * 0123456789abc
>          * AA B       CC
>          */
>         vma_a = alloc_and_link_vma(&mm, 0, 0x2000, 0, vm_flags);
>         ASSERT_NE(vma_a, NULL);
> +       if (a_is_sticky)
> +               vm_flags_set(vma_a, VM_STICKY);
>         /* We give each VMA a single avc so we can test anon_vma duplication. */
>         INIT_LIST_HEAD(&vma_a->anon_vma_chain);
>         list_add(&dummy_anon_vma_chain_a.same_vma, &vma_a->anon_vma_chain);
>
>         vma_b = alloc_and_link_vma(&mm, 0x3000, 0x4000, 3, vm_flags);
>         ASSERT_NE(vma_b, NULL);
> +       if (b_is_sticky)
> +               vm_flags_set(vma_b, VM_STICKY);
>         INIT_LIST_HEAD(&vma_b->anon_vma_chain);
>         list_add(&dummy_anon_vma_chain_b.same_vma, &vma_b->anon_vma_chain);
>
>         vma_c = alloc_and_link_vma(&mm, 0xb000, 0xc000, 0xb, vm_flags);
>         ASSERT_NE(vma_c, NULL);
> +       if (c_is_sticky)
> +               vm_flags_set(vma_c, VM_STICKY);
>         INIT_LIST_HEAD(&vma_c->anon_vma_chain);
>         list_add(&dummy_anon_vma_chain_c.same_vma, &vma_c->anon_vma_chain);
>
> @@ -520,6 +531,8 @@ static bool test_merge_new(void)
>         ASSERT_EQ(vma->anon_vma, &dummy_anon_vma);
>         ASSERT_TRUE(vma_write_started(vma));
>         ASSERT_EQ(mm.map_count, 3);
> +       if (is_sticky || a_is_sticky || b_is_sticky)
> +               ASSERT_TRUE(IS_SET(vma->vm_flags, VM_STICKY));
>
>         /*
>          * Merge to PREVIOUS VMA.
> @@ -537,6 +550,8 @@ static bool test_merge_new(void)
>         ASSERT_EQ(vma->anon_vma, &dummy_anon_vma);
>         ASSERT_TRUE(vma_write_started(vma));
>         ASSERT_EQ(mm.map_count, 3);
> +       if (is_sticky || a_is_sticky)
> +               ASSERT_TRUE(IS_SET(vma->vm_flags, VM_STICKY));
>
>         /*
>          * Merge to NEXT VMA.
> @@ -556,6 +571,8 @@ static bool test_merge_new(void)
>         ASSERT_EQ(vma->anon_vma, &dummy_anon_vma);
>         ASSERT_TRUE(vma_write_started(vma));
>         ASSERT_EQ(mm.map_count, 3);
> +       if (is_sticky) /* D uses is_sticky. */
> +               ASSERT_TRUE(IS_SET(vma->vm_flags, VM_STICKY));
>
>         /*
>          * Merge BOTH sides.
> @@ -574,6 +591,8 @@ static bool test_merge_new(void)
>         ASSERT_EQ(vma->anon_vma, &dummy_anon_vma);
>         ASSERT_TRUE(vma_write_started(vma));
>         ASSERT_EQ(mm.map_count, 2);
> +       if (is_sticky || a_is_sticky)
> +               ASSERT_TRUE(IS_SET(vma->vm_flags, VM_STICKY));
>
>         /*
>          * Merge to NEXT VMA.
> @@ -592,6 +611,8 @@ static bool test_merge_new(void)
>         ASSERT_EQ(vma->anon_vma, &dummy_anon_vma);
>         ASSERT_TRUE(vma_write_started(vma));
>         ASSERT_EQ(mm.map_count, 2);
> +       if (is_sticky || c_is_sticky)
> +               ASSERT_TRUE(IS_SET(vma->vm_flags, VM_STICKY));
>
>         /*
>          * Merge BOTH sides.
> @@ -609,6 +630,8 @@ static bool test_merge_new(void)
>         ASSERT_EQ(vma->anon_vma, &dummy_anon_vma);
>         ASSERT_TRUE(vma_write_started(vma));
>         ASSERT_EQ(mm.map_count, 1);
> +       if (is_sticky || a_is_sticky || c_is_sticky)
> +               ASSERT_TRUE(IS_SET(vma->vm_flags, VM_STICKY));
>
>         /*
>          * Final state.
> @@ -637,6 +660,20 @@ static bool test_merge_new(void)
>         return true;
>  }
>
> +static bool test_merge_new(void)
> +{
> +       int i, j, k, l;
> +
> +       /* Generate every possible permutation of sticky flags. */
> +       for (i = 0; i < 2; i++)
> +               for (j = 0; j < 2; j++)
> +                       for (k = 0; k < 2; k++)
> +                               for (l = 0; l < 2; l++)
> +                                       ASSERT_TRUE(__test_merge_new(i, j, k, l));
> +
> +       return true;
> +}
> +
>  static bool test_vma_merge_special_flags(void)
>  {
>         vm_flags_t vm_flags = VM_READ | VM_WRITE | VM_MAYREAD | VM_MAYWRITE;
> @@ -973,9 +1010,11 @@ static bool test_vma_merge_new_with_close(void)
>         return true;
>  }
>
> -static bool test_merge_existing(void)
> +static bool __test_merge_existing(bool prev_is_sticky, bool middle_is_sticky, bool next_is_sticky)
>  {
>         vm_flags_t vm_flags = VM_READ | VM_WRITE | VM_MAYREAD | VM_MAYWRITE;
> +       vm_flags_t prev_flags = vm_flags;
> +       vm_flags_t next_flags = vm_flags;
>         struct mm_struct mm = {};
>         VMA_ITERATOR(vmi, &mm, 0);
>         struct vm_area_struct *vma, *vma_prev, *vma_next;
> @@ -988,6 +1027,13 @@ static bool test_merge_existing(void)
>         };
>         struct anon_vma_chain avc = {};
>
> +       if (prev_is_sticky)
> +               prev_flags |= VM_STICKY;
> +       if (middle_is_sticky)
> +               vm_flags |= VM_STICKY;
> +       if (next_is_sticky)
> +               next_flags |= VM_STICKY;
> +
>         /*
>          * Merge right case - partial span.
>          *
> @@ -1000,7 +1046,7 @@ static bool test_merge_existing(void)
>          */
>         vma = alloc_and_link_vma(&mm, 0x2000, 0x6000, 2, vm_flags);
>         vma->vm_ops = &vm_ops; /* This should have no impact. */
> -       vma_next = alloc_and_link_vma(&mm, 0x6000, 0x9000, 6, vm_flags);
> +       vma_next = alloc_and_link_vma(&mm, 0x6000, 0x9000, 6, next_flags);
>         vma_next->vm_ops = &vm_ops; /* This should have no impact. */
>         vmg_set_range_anon_vma(&vmg, 0x3000, 0x6000, 3, vm_flags, &dummy_anon_vma);
>         vmg.middle = vma;
> @@ -1018,6 +1064,8 @@ static bool test_merge_existing(void)
>         ASSERT_TRUE(vma_write_started(vma));
>         ASSERT_TRUE(vma_write_started(vma_next));
>         ASSERT_EQ(mm.map_count, 2);
> +       if (middle_is_sticky || next_is_sticky)
> +               ASSERT_TRUE(IS_SET(vma_next->vm_flags, VM_STICKY));
>
>         /* Clear down and reset. */
>         ASSERT_EQ(cleanup_mm(&mm, &vmi), 2);
> @@ -1033,7 +1081,7 @@ static bool test_merge_existing(void)
>          *   NNNNNNN
>          */
>         vma = alloc_and_link_vma(&mm, 0x2000, 0x6000, 2, vm_flags);
> -       vma_next = alloc_and_link_vma(&mm, 0x6000, 0x9000, 6, vm_flags);
> +       vma_next = alloc_and_link_vma(&mm, 0x6000, 0x9000, 6, next_flags);
>         vma_next->vm_ops = &vm_ops; /* This should have no impact. */
>         vmg_set_range_anon_vma(&vmg, 0x2000, 0x6000, 2, vm_flags, &dummy_anon_vma);
>         vmg.middle = vma;
> @@ -1046,6 +1094,8 @@ static bool test_merge_existing(void)
>         ASSERT_EQ(vma_next->anon_vma, &dummy_anon_vma);
>         ASSERT_TRUE(vma_write_started(vma_next));
>         ASSERT_EQ(mm.map_count, 1);
> +       if (middle_is_sticky || next_is_sticky)
> +               ASSERT_TRUE(IS_SET(vma_next->vm_flags, VM_STICKY));
>
>         /* Clear down and reset. We should have deleted vma. */
>         ASSERT_EQ(cleanup_mm(&mm, &vmi), 1);
> @@ -1060,7 +1110,7 @@ static bool test_merge_existing(void)
>          * 0123456789
>          * PPPPPPV
>          */
> -       vma_prev = alloc_and_link_vma(&mm, 0, 0x3000, 0, vm_flags);
> +       vma_prev = alloc_and_link_vma(&mm, 0, 0x3000, 0, prev_flags);
>         vma_prev->vm_ops = &vm_ops; /* This should have no impact. */
>         vma = alloc_and_link_vma(&mm, 0x3000, 0x7000, 3, vm_flags);
>         vma->vm_ops = &vm_ops; /* This should have no impact. */
> @@ -1080,6 +1130,8 @@ static bool test_merge_existing(void)
>         ASSERT_TRUE(vma_write_started(vma_prev));
>         ASSERT_TRUE(vma_write_started(vma));
>         ASSERT_EQ(mm.map_count, 2);
> +       if (prev_is_sticky || middle_is_sticky)
> +               ASSERT_TRUE(IS_SET(vma_prev->vm_flags, VM_STICKY));
>
>         /* Clear down and reset. */
>         ASSERT_EQ(cleanup_mm(&mm, &vmi), 2);
> @@ -1094,7 +1146,7 @@ static bool test_merge_existing(void)
>          * 0123456789
>          * PPPPPPP
>          */
> -       vma_prev = alloc_and_link_vma(&mm, 0, 0x3000, 0, vm_flags);
> +       vma_prev = alloc_and_link_vma(&mm, 0, 0x3000, 0, prev_flags);
>         vma_prev->vm_ops = &vm_ops; /* This should have no impact. */
>         vma = alloc_and_link_vma(&mm, 0x3000, 0x7000, 3, vm_flags);
>         vmg_set_range_anon_vma(&vmg, 0x3000, 0x7000, 3, vm_flags, &dummy_anon_vma);
> @@ -1109,6 +1161,8 @@ static bool test_merge_existing(void)
>         ASSERT_EQ(vma_prev->anon_vma, &dummy_anon_vma);
>         ASSERT_TRUE(vma_write_started(vma_prev));
>         ASSERT_EQ(mm.map_count, 1);
> +       if (prev_is_sticky || middle_is_sticky)
> +               ASSERT_TRUE(IS_SET(vma_prev->vm_flags, VM_STICKY));
>
>         /* Clear down and reset. We should have deleted vma. */
>         ASSERT_EQ(cleanup_mm(&mm, &vmi), 1);
> @@ -1123,10 +1177,10 @@ static bool test_merge_existing(void)
>          * 0123456789
>          * PPPPPPPPPP
>          */
> -       vma_prev = alloc_and_link_vma(&mm, 0, 0x3000, 0, vm_flags);
> +       vma_prev = alloc_and_link_vma(&mm, 0, 0x3000, 0, prev_flags);
>         vma_prev->vm_ops = &vm_ops; /* This should have no impact. */
>         vma = alloc_and_link_vma(&mm, 0x3000, 0x7000, 3, vm_flags);
> -       vma_next = alloc_and_link_vma(&mm, 0x7000, 0x9000, 7, vm_flags);
> +       vma_next = alloc_and_link_vma(&mm, 0x7000, 0x9000, 7, next_flags);
>         vmg_set_range_anon_vma(&vmg, 0x3000, 0x7000, 3, vm_flags, &dummy_anon_vma);
>         vmg.prev = vma_prev;
>         vmg.middle = vma;
> @@ -1139,6 +1193,8 @@ static bool test_merge_existing(void)
>         ASSERT_EQ(vma_prev->anon_vma, &dummy_anon_vma);
>         ASSERT_TRUE(vma_write_started(vma_prev));
>         ASSERT_EQ(mm.map_count, 1);
> +       if (prev_is_sticky || middle_is_sticky || next_is_sticky)
> +               ASSERT_TRUE(IS_SET(vma_prev->vm_flags, VM_STICKY));
>
>         /* Clear down and reset. We should have deleted prev and next. */
>         ASSERT_EQ(cleanup_mm(&mm, &vmi), 1);
> @@ -1158,9 +1214,9 @@ static bool test_merge_existing(void)
>          * PPPVVVVVNNN
>          */
>
> -       vma_prev = alloc_and_link_vma(&mm, 0, 0x3000, 0, vm_flags);
> +       vma_prev = alloc_and_link_vma(&mm, 0, 0x3000, 0, prev_flags);
>         vma = alloc_and_link_vma(&mm, 0x3000, 0x8000, 3, vm_flags);
> -       vma_next = alloc_and_link_vma(&mm, 0x8000, 0xa000, 8, vm_flags);
> +       vma_next = alloc_and_link_vma(&mm, 0x8000, 0xa000, 8, next_flags);
>
>         vmg_set_range(&vmg, 0x4000, 0x5000, 4, vm_flags);
>         vmg.prev = vma;
> @@ -1203,6 +1259,19 @@ static bool test_merge_existing(void)
>         return true;
>  }
>
> +static bool test_merge_existing(void)
> +{
> +       int i, j, k;
> +
> +       /* Generate every possible permutation of sticky flags. */
> +       for (i = 0; i < 2; i++)
> +               for (j = 0; j < 2; j++)
> +                       for (k = 0; k < 2; k++)
> +                               ASSERT_TRUE(__test_merge_existing(i, j, k));
> +
> +       return true;
> +}
> +
>  static bool test_anon_vma_non_mergeable(void)
>  {
>         vm_flags_t vm_flags = VM_READ | VM_WRITE | VM_MAYREAD | VM_MAYWRITE;
> diff --git a/tools/testing/vma/vma_internal.h b/tools/testing/vma/vma_internal.h
> index e40c93edc5a7..3d9cb3a9411a 100644
> --- a/tools/testing/vma/vma_internal.h
> +++ b/tools/testing/vma/vma_internal.h
> @@ -117,6 +117,38 @@ extern unsigned long dac_mmap_min_addr;
>  #define VM_SEALED      VM_NONE
>  #endif
>
> +/* Flags which should result in page tables being copied on fork. */
> +#define VM_COPY_ON_FORK VM_MAYBE_GUARD
> +
> +/*
> + * Flags which should be 'sticky' on merge - that is, flags which, when one VMA
> + * possesses it but the other does not, the merged VMA should nonetheless have
> + * applied to it:
> + *
> + * VM_COPY_ON_FORK - These flags indicates that a VMA maps a range that contains
> + *                   metadata which should be unconditionally propagated upon
> + *                   fork. When merging two VMAs, we encapsulate this range in
> + *                   the merged VMA, so the flag should be 'sticky' as a result.
> + */
> +#define VM_STICKY VM_COPY_ON_FORK
> +
> +/*
> + * VMA flags we ignore for the purposes of merge, i.e. one VMA possessing one
> + * of these flags and the other not does not preclude a merge.
> + *
> + * VM_SOFTDIRTY - Should not prevent from VMA merging, if we match the flags but
> + *                dirty bit -- the caller should mark merged VMA as dirty. If
> + *                dirty bit won't be excluded from comparison, we increase
> + *                pressure on the memory system forcing the kernel to generate
> + *                new VMAs when old one could be extended instead.
> + *
> + *    VM_STICKY - If one VMA has flags which must be 'sticky', that is ones
> + *                which should propagate to all VMAs, but the other does not,
> + *                the merge should still proceed with the merge logic applying
> + *                sticky flags to the final VMA.
> + */
> +#define VM_IGNORE_MERGE (VM_SOFTDIRTY | VM_STICKY)
> +
>  #define FIRST_USER_ADDRESS     0UL
>  #define USER_PGTABLES_CEILING  0UL
>
> --
> 2.51.0
>