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Message-ID: <CAJuCfpG=D_MbZyauk2WJZ4evvoUQ0XEXwhwYpqS8rXqCGfVEng@mail.gmail.com>
Date: Tue, 30 Dec 2025 14:02:29 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, "Liam R . Howlett" <Liam.Howlett@...cle.com>,
Vlastimil Babka <vbabka@...e.cz>, Shakeel Butt <shakeel.butt@...ux.dev>,
David Hildenbrand <david@...nel.org>, Rik van Riel <riel@...riel.com>, Harry Yoo <harry.yoo@...cle.com>,
Jann Horn <jannh@...gle.com>, Mike Rapoport <rppt@...nel.org>, Michal Hocko <mhocko@...e.com>,
Pedro Falcato <pfalcato@...e.de>, Chris Li <chriscli@...gle.com>,
Barry Song <v-songbaohua@...o.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 8/8] mm/rmap: separate out fork-only logic on anon_vma_clone()
On Wed, Dec 17, 2025 at 4:27 AM Lorenzo Stoakes
<lorenzo.stoakes@...cle.com> wrote:
>
> Specify which operation is being performed to anon_vma_clone(), which
> allows us to do checks specific to each operation type, as well as to
> separate out and make clear that the anon_vma reuse logic is absolutely
> specific to fork only.
>
> This opens the door to further refactorings and refinements later as we
> have more information to work with.
>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> ---
> mm/internal.h | 11 +++++-
> mm/rmap.c | 67 ++++++++++++++++++++++----------
> mm/vma.c | 6 +--
> tools/testing/vma/vma_internal.h | 11 +++++-
> 4 files changed, 69 insertions(+), 26 deletions(-)
>
> diff --git a/mm/internal.h b/mm/internal.h
> index 469d4ef1ccc5..b4d4bca0f9a7 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -244,7 +244,16 @@ static inline void anon_vma_unlock_read(struct anon_vma *anon_vma)
>
> struct anon_vma *folio_get_anon_vma(const struct folio *folio);
>
> -int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src);
> +/* Operations which modify VMAs. */
> +enum vma_operation {
> + VMA_OP_SPLIT,
> + VMA_OP_MERGE_UNFAULTED,
> + VMA_OP_REMAP,
> + VMA_OP_FORK,
> +};
> +
> +int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src,
> + enum vma_operation operation);
> int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma);
> int __anon_vma_prepare(struct vm_area_struct *vma);
> void unlink_anon_vmas(struct vm_area_struct *vma);
> diff --git a/mm/rmap.c b/mm/rmap.c
> index de9de6d71c23..f08e6bc57379 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -232,12 +232,13 @@ int __anon_vma_prepare(struct vm_area_struct *vma)
> }
>
> static void check_anon_vma_clone(struct vm_area_struct *dst,
> - struct vm_area_struct *src)
> + struct vm_area_struct *src,
> + enum vma_operation operation)
> {
> /* The write lock must be held. */
> mmap_assert_write_locked(src->vm_mm);
> - /* If not a fork (implied by dst->anon_vma) then must be on same mm. */
> - VM_WARN_ON_ONCE(dst->anon_vma && dst->vm_mm != src->vm_mm);
> + /* If not a fork then must be on same mm. */
> + VM_WARN_ON_ONCE(operation != VMA_OP_FORK && dst->vm_mm != src->vm_mm);
>
> /* No source anon_vma is a no-op. */
> VM_WARN_ON_ONCE(!src->anon_vma && !list_empty(&src->anon_vma_chain));
> @@ -249,6 +250,35 @@ static void check_anon_vma_clone(struct vm_area_struct *dst,
> * must be the same across dst and src.
> */
> VM_WARN_ON_ONCE(dst->anon_vma && dst->anon_vma != src->anon_vma);
> +
> + /* For the anon_vma to be compatible, it can only be singular. */
> + VM_WARN_ON_ONCE(operation == VMA_OP_MERGE_UNFAULTED &&
> + !list_is_singular(&src->anon_vma_chain));
> +#ifdef CONFIG_PER_VMA_LOCK
> + /* Only merging an unfaulted VMA leaves the destination attached. */
Do you mean to say that for all other operations dst should not be yet
attached? If so, "leaves the destination attached" is a bit confusing
choice of words to indicate that attaching the VMA in other cases has
not yet happened. Maybe reword as: "For cases other than merging an
unfaulted VMA, the VMA should not be attached yet."
> + VM_WARN_ON_ONCE(operation != VMA_OP_MERGE_UNFAULTED &&
> + vma_is_attached(dst));
> +#endif
> +}
> +
> +static void find_reusable_anon_vma(struct vm_area_struct *vma,
> + struct anon_vma *anon_vma)
Naming is hard but a function that assigns vma->anon_vma and
increments num_active_vmas should not be called
"find_reusable_anon_vma". I would suggest keeping the name but making
it return the anon_vma (or NULL if it's not reusable), letting the
caller do the assignment and the increment.
> +{
> + /* If already populated, nothing to do.*/
> + if (vma->anon_vma)
> + return;
> +
> + /*
> + * We reuse an anon_vma if any linking VMAs were unmapped and it has
> + * only a single child at most.
> + */
> + if (anon_vma->num_active_vmas > 0)
> + return;
> + if (anon_vma->num_children > 1)
> + return;
> +
> + vma->anon_vma = anon_vma;
> + anon_vma->num_active_vmas++;
You moved num_active_vmas++ here to fix the accounting later... Sneaky
but correct AFAICT :)
> }
>
> /**
> @@ -256,6 +286,7 @@ static void check_anon_vma_clone(struct vm_area_struct *dst,
> * all of the anon_vma objects contained within @src anon_vma_chain's.
> * @dst: The destination VMA with an empty anon_vma_chain.
> * @src: The source VMA we wish to duplicate.
> + * @operation: The type of operation which resulted in the clone.
> *
> * This is the heart of the VMA side of the anon_vma implementation - we invoke
> * this function whenever we need to set up a new VMA's anon_vma state.
> @@ -278,14 +309,16 @@ static void check_anon_vma_clone(struct vm_area_struct *dst,
> *
> * Returns: 0 on success, -ENOMEM on failure.
> */
> -int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
> +int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src,
> + enum vma_operation operation)
> {
> struct anon_vma_chain *avc, *pavc;
> + struct anon_vma *active_anon_vma = src->anon_vma;
>
> - if (!src->anon_vma)
> + if (!active_anon_vma)
> return 0;
>
> - check_anon_vma_clone(dst, src);
> + check_anon_vma_clone(dst, src, operation);
>
> /*
> * Allocate AVCs. We don't need an anon_vma lock for this as we
> @@ -309,22 +342,14 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
> struct anon_vma *anon_vma = avc->anon_vma;
>
> anon_vma_interval_tree_insert(avc, &anon_vma->rb_root);
> -
> - /*
> - * Reuse existing anon_vma if it has no vma and only one
> - * anon_vma child.
> - *
> - * Root anon_vma is never reused:
> - * it has self-parent reference and at least one child.
> - */
> - if (!dst->anon_vma && src->anon_vma &&
> - anon_vma->num_children < 2 &&
> - anon_vma->num_active_vmas == 0)
> - dst->anon_vma = anon_vma;
> + if (operation == VMA_OP_FORK)
> + find_reusable_anon_vma(dst, anon_vma);
> }
> - if (dst->anon_vma)
> +
> + if (operation != VMA_OP_FORK)
> dst->anon_vma->num_active_vmas++;
> - anon_vma_unlock_write(src->anon_vma);
> +
> + anon_vma_unlock_write(active_anon_vma);
> return 0;
>
> enomem_failure:
> @@ -361,7 +386,7 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
> * First, attach the new VMA to the parent VMA's anon_vmas,
> * so rmap can find non-COWed pages in child processes.
> */
> - error = anon_vma_clone(vma, pvma);
> + error = anon_vma_clone(vma, pvma, VMA_OP_FORK);
> if (error)
> return error;
>
> diff --git a/mm/vma.c b/mm/vma.c
> index feb4bbd3b259..e297c3a94133 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -525,7 +525,7 @@ __split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
> if (err)
> goto out_free_vmi;
>
> - err = anon_vma_clone(new, vma);
> + err = anon_vma_clone(new, vma, VMA_OP_SPLIT);
> if (err)
> goto out_free_mpol;
>
> @@ -623,7 +623,7 @@ static int dup_anon_vma(struct vm_area_struct *dst,
>
> vma_assert_write_locked(dst);
> dst->anon_vma = src->anon_vma;
> - ret = anon_vma_clone(dst, src);
> + ret = anon_vma_clone(dst, src, VMA_OP_MERGE_UNFAULTED);
> if (ret)
> return ret;
>
> @@ -1862,7 +1862,7 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
> vma_set_range(new_vma, addr, addr + len, pgoff);
> if (vma_dup_policy(vma, new_vma))
> goto out_free_vma;
> - if (anon_vma_clone(new_vma, vma))
> + if (anon_vma_clone(new_vma, vma, VMA_OP_REMAP))
> goto out_free_mempol;
> if (new_vma->vm_file)
> get_file(new_vma->vm_file);
> diff --git a/tools/testing/vma/vma_internal.h b/tools/testing/vma/vma_internal.h
> index 93e5792306d9..7fa56dcc53a6 100644
> --- a/tools/testing/vma/vma_internal.h
> +++ b/tools/testing/vma/vma_internal.h
> @@ -600,6 +600,14 @@ struct mmap_action {
> bool hide_from_rmap_until_complete :1;
> };
>
> +/* Operations which modify VMAs. */
> +enum vma_operation {
> + VMA_OP_SPLIT,
> + VMA_OP_MERGE_UNFAULTED,
> + VMA_OP_REMAP,
> + VMA_OP_FORK,
> +};
> +
> /*
> * Describes a VMA that is about to be mmap()'ed. Drivers may choose to
> * manipulate mutable fields which will cause those fields to be updated in the
> @@ -1157,7 +1165,8 @@ static inline int vma_dup_policy(struct vm_area_struct *src, struct vm_area_stru
> return 0;
> }
>
> -static inline int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
> +static inline int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src,
> + enum vma_operation operation)
> {
> /* For testing purposes. We indicate that an anon_vma has been cloned. */
> if (src->anon_vma != NULL) {
> --
> 2.52.0
>
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