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Message-ID: <21daf689-89c5-43a6-b83b-2dc405a2fb57@lucifer.local>
Date: Tue, 6 Jan 2026 14:43:21 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Suren Baghdasaryan <surenb@...gle.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 Tue, Dec 30, 2025 at 02:02:29PM -0800, Suren Baghdasaryan wrote:
> 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.
I mean totally agree this name is bad actually :) good spot.
Since you might already have populated so we can't return NULL, so I think a
rename is better here actually -> try_to_reuse_anon_vma() which more accurately
describes what's going on and takes into account the fact it's doing a stats
update also!
>
> > +{
> > + /* 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 :)
;)
I would say in my defence (perhaps unnecessarily, as being sneaky is a key
kernel dev skill imo ;) that it's no more sneaky than the code already
was. Probably :)
Really this is about making the situation clearer in anon_vma_clone(). We could
instead have:
if (dst->anon_vma)
dst->anon_vma->num_active_vmas++;
And with the assignment here, we cover off the fork + reuse case.
However this doesn't make it clear that in fact for all non-fork cases we are
_always_ incrementing this value, which is what I wanted to make super obvious.
One thing here that I think would help is to rename vma to dst just to
carry that across from anon_vma_clone() and to make it clear we're
checking/manipulating the destination VMA here.
>
> > }
> >
> > /**
> > @@ -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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