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Message-ID: <49f89731-ba74-49b6-bf08-7cb5d2a2376a@lucifer.local>
Date: Thu, 30 Oct 2025 08:25:39 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Jonathan Corbet <corbet@....net>, David Hildenbrand <david@...hat.com>,
"Liam R . Howlett" <Liam.Howlett@...cle.com>,
Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
Michal Hocko <mhocko@...e.com>, Steven Rostedt <rostedt@...dmis.org>,
Masami Hiramatsu <mhiramat@...nel.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Jann Horn <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>,
linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-doc@...r.kernel.org, linux-mm@...ck.org,
linux-trace-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
Andrei Vagin <avagin@...il.com>
Subject: Re: [PATCH 2/3] mm: implement sticky, copy on fork VMA flags
On Wed, Oct 29, 2025 at 09:35:25PM -0700, Suren Baghdasaryan wrote:
> On Wed, Oct 29, 2025 at 9:51 AM Lorenzo Stoakes
> <lorenzo.stoakes@...cle.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"?
Yeah... typo bonanza this series :) will fix.
>
> >
> > 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
Haha good lord. Will fix also!
>
> > 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@...cle.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.
Hm I thought this was implied. Will update to be clear however!
>
> > + */
> > +#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 ?
I most learn to not typo so much :)
Yes you're right, will fix! :P
>
> > + * 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.
Yeah can split it out! I do tend to do that actually, not sure why I
deviated from that here.
>
> > @@ -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
> >
Thanks for review!
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