Re: [PATCH v4 3/5] mm: Add RCU-based VMA lookup helper that waits for writers

[Suren Baghdasaryan]

On Mon, Aug 10, 2026 at 3:42 AM Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx> wrote:
>
> On Thu, Aug 06, 2026 at 01:05:46PM -0700, Suren Baghdasaryan wrote:
> > From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> >
> > == Background ==
> >
> > There are basically two parallel ways to look up a VMA: the
> > traditional way, which is protected by mmap_read_lock, and the RCU-based
> > per-VMA lock way which is based on RCU and refcounts.
> >
> > == Problem ==
> >
> > The mmap_lock one is more straightforward to use but it has a big
> > disadvantage in that it can not be mixed with page faults since those
> > can take mmap_lock for read, which can deadlock when mixed with nested
> > page faults and parallel writers.
> > For example:
> >
> >       mmap_read_lock(mm);
> >       // Another thread does mmap_write_lock().
> >       // New mmap_lock readers are blocked.
> >       vma = vma_lookup(mm, address);
> >       // This deadlocks on mmap_read_lock() if it faults:
> >       copy_from_user(address);
> >       mmap_read_unlock(mm);
> >
> > The per-VMA lock can be mixed with faults, but they can fail and need to
> > be able to fall back to the traditional way.
>
> As discussed in reply to Matthew, this isn't so useful to have here I don't
> think.

Ack.

>
> >
> > == Solution ==
> >
> > Add vma_start_read_unlocked() - a variant of the RCU-based lookup that
> > waits for writers. This is basically the same as the existing RCU-based
> > lookup, but on a failure to lock it temporarily takes mmap_lock for read
> > and waits for writers to finish before locking the VMA, dropping the
> > mmap_lock and returning the locked VMA. This has some advantages:
>
> Nitty but maybe reads better as something like:
>
>         ..., on failing to acquire the VMA lock it waits for writers to
>         finish by temporarily acquiring the mmap read lock and taking the
>         VMA lock with it held via vma_start_read_locked() before dropping
>         it again.

Sounds good. Will adopt your version/

>
> >
> >  1. Callers do not need to have a fallback path for when they
> >     collide with writers.
> >  2. It can be used in contexts where page faults can happen because
> >     it can take the mmap_lock for read but never *holds* it.
> >  3. Its fast path does not require taking mmap_lock for read.
> >
> > Basically, when applied correctly, this approach results in faster
> > *and* simpler code.
>
> I think this suffices as justification for the function with the above
> deadlock scenario simply excised.

Ack.

>
> >
> > While at it, fix the comments for vma_start_read_locked(),
> > vma_start_read_locked_nested(), and uffd_lock_vma().
> >
> > Suggested-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
>
> Thanks :>)
>
> > Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
>
> LGTM, so with the changes to commit message applied feel free to add:
>
> Reviewed-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>

Thanks!

>
> > Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx>
> > Cc: Lorenzo Stoakes <ljs@xxxxxxxxxx>
> > Cc: Vlastimil Babka <vbabka@xxxxxxxxxx>
> > Cc: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> > Cc: linux-mm@xxxxxxxxx
> > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > Cc: Arve Hjønnevåg <arve@xxxxxxxxxxx>
> > Cc: Todd Kjos <tkjos@xxxxxxxxxxx>
> > Cc: Christian Brauner <christian@xxxxxxxxxx>
> > Cc: Carlos Llamas <cmllamas@xxxxxxxxxx>
> > Cc: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> > Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
> > Cc: David Ahern <dsahern@xxxxxxxxxx>
> > Cc: netdev@xxxxxxxxxxxxxxx
> > ---
> >  include/linux/mmap_lock.h | 19 +++++++++++++++----
> >  mm/mmap_lock.c            | 35 +++++++++++++++++++++++++++++++++++
> >  mm/userfaultfd.c          |  6 ++++--
> >  3 files changed, 54 insertions(+), 6 deletions(-)
> >
> > diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
> > index 7b2bbb09a952..a23fe6cbe301 100644
> > --- a/include/linux/mmap_lock.h
> > +++ b/include/linux/mmap_lock.h
> > @@ -228,10 +228,14 @@ static inline void vma_refcount_put(struct vm_area_struct *vma)
> >  }
> >
> >  /*
> > - * Use only while holding mmap read lock which guarantees that locking will not
> > - * fail (nobody can concurrently write-lock the vma). vma_start_read() should
> > + * Use only while holding mmap read lock which guarantees that vma lock is not
> > + * contended (nobody can concurrently write-lock the vma). vma_start_read() should
> >   * not be used in such cases because it might fail due to mm_lock_seq overflow.
> >   * This functionality is used to obtain vma read lock and drop the mmap read lock.
> > + *
> > + * VMA can't be detached while we are holding mmap lock, therefore in practice this
> > + * function can fail only when there are so many readers that vm_refcnt overflows.
> > + * The failure case is very unlikely and is already annotated as such internally.
> >   */
> >  static inline bool vma_start_read_locked_nested(struct vm_area_struct *vma, int subclass)
> >  {
> > @@ -247,16 +251,23 @@ static inline bool vma_start_read_locked_nested(struct vm_area_struct *vma, int
> >  }
> >
> >  /*
> > - * Use only while holding mmap read lock which guarantees that locking will not
> > - * fail (nobody can concurrently write-lock the vma). vma_start_read() should
> > + * Use only while holding mmap read lock which guarantees that vma lock is not
> > + * contended (nobody can concurrently write-lock the vma). vma_start_read() should
> >   * not be used in such cases because it might fail due to mm_lock_seq overflow.
> >   * This functionality is used to obtain vma read lock and drop the mmap read lock.
> > + *
> > + * VMA can't be detached while we are holding mmap lock, therefore in practice this
> > + * function can fail only when there are so many readers that vm_refcnt overflows.
> > + * The failure case is very unlikely and is already annotated as such internally.
> >   */
> >  static inline bool vma_start_read_locked(struct vm_area_struct *vma)
> >  {
> >       return vma_start_read_locked_nested(vma, 0);
> >  }
> >
> > +struct vm_area_struct *vma_start_read_unlocked(struct mm_struct *mm,
> > +                                            unsigned long address);
> > +
> >  static inline void vma_end_read(struct vm_area_struct *vma)
> >  {
> >       vma_refcount_put(vma);
> > diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c
> > index e20d01e8d38f..1c4902131e98 100644
> > --- a/mm/mmap_lock.c
> > +++ b/mm/mmap_lock.c
> > @@ -338,6 +338,41 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
> >       return NULL;
> >  }
> >
> > +/**
> > + * vma_start_read_unlocked() - Find the VMA covering 'address' and read-lock it.
> > + * @mm: the mm_struct of the address space to search
> > + * @address: address that the vma should contain
> > + *
> > + * The fast path does not take mmap_lock. Waits for writers to finish if the
> > + * VMA is being modified by taking mmap_lock.
> > + * Use when mmap_lock is not held, otherwise use vma_start_read_locked().
> > + * Nothing prevents VMAs being unmapped/mapped before or after the VMA is
> > + * looked up, if a stronger guarantee is required, take an mmap_lock.
>
> Great thanks!
>
> > + *
> > + * Return: If a VMA exists which spans @address, return that VMA, read-locked.
> > + * If no VMA is mapped there or, very unlikely, a reference count overflow
> > + * occurred, return NULL.
> > + */
> > +struct vm_area_struct *vma_start_read_unlocked(struct mm_struct *mm,
> > +                                            unsigned long address)
> > +{
> > +     struct vm_area_struct *vma;
> > +
> > +     /* Fast path: return stable VMA covering 'address': */
> > +     vma = lock_vma_under_rcu(mm, address);
> > +     if (vma)
> > +             return vma;
> > +
> > +     /* Slow path: preclude VMA writers by temporarily getting mmap read lock. */
> > +     mmap_read_lock(mm);
> > +     vma = vma_lookup(mm, address);
> > +     if (vma && !vma_start_read_locked(vma))
> > +             vma = NULL;
> > +     mmap_read_unlock(mm);
> > +
> > +     return vma;
> > +}
> > +
> >  static struct vm_area_struct *lock_next_vma_under_mmap_lock(struct mm_struct *mm,
> >                                                           struct vma_iterator *vmi,
> >                                                           unsigned long from_addr)
> > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > index edd90892f8cc..c3a0c38a3dc3 100644
> > --- a/mm/userfaultfd.c
> > +++ b/mm/userfaultfd.c
> > @@ -129,8 +129,10 @@ struct vm_area_struct *find_vma_and_prepare_anon(struct mm_struct *mm,
> >   *
> >   * Should be called without holding mmap_lock.
> >   *
> > - * Return: A locked vma containing @address, -ENOENT if no vma is found, or
> > - * -ENOMEM if anon_vma couldn't be allocated.
> > + * Return: A locked vma containing @address, -ENOENT if no vma is found,
> > + * -ENOMEM if anon_vma couldn't be allocated, or -EAGAIN if vma refcount
> > + * overflow happened due to high number of readers and the caller should
> > + * retry later.
> >   */
> >  static struct vm_area_struct *uffd_lock_vma(struct mm_struct *mm,
> >                                      unsigned long address)
> > --
> > 2.55.0.654.g21b8a5bc05-goog
> >
>
> --
> Cheers, Lorenzo