Re: [PATCH v3 3/5] mm: Add RCU-based VMA lookup helper that waits for writers

[Suren Baghdasaryan]

On Mon, Aug 3, 2026 at 9:44 AM Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx> wrote:
>
> On Mon, Aug 03, 2026 at 06:24:34PM +0200, Vlastimil Babka (SUSE) wrote:
> > On 8/3/26 17:00, Lorenzo Stoakes (ARM) wrote:
> > > On Mon, Aug 03, 2026 at 04:55:19PM +0200, Vlastimil Babka (SUSE) wrote:
> > >> On 8/2/26 23:54, 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.
> > >> >
> > >> > == Solution ==
> > >> >
> > >> > Add 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:
> > >>
> > >> Maybe mention that the helper is called vma_start_read_unlocked()?

Ack.

> > >>
> > >> >
> > >> >  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.
> > >> >
> > >> > Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > >> > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > >> > 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 | 15 +++++++++++----
> > >> >  mm/mmap_lock.c            | 29 +++++++++++++++++++++++++++++
> > >> >  mm/userfaultfd.c          |  6 ++++--
> > >> >  3 files changed, 44 insertions(+), 6 deletions(-)
> > >> >
> > >> > diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
> > >> > index eb32b482434e..fdd8f5cf5722 100644
> > >> > --- a/include/linux/mmap_lock.h
> > >> > +++ b/include/linux/mmap_lock.h
> > >> > @@ -228,10 +228,12 @@ 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.
> > >> >   */
> > >> >  static inline bool vma_start_read_locked_nested(struct vm_area_struct *vma, int subclass)
> > >> >  {
> > >> > @@ -247,16 +249,21 @@ 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.
> > >> >   */
> > >> >  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..6ff05e68e61b 100644
> > >> > --- a/mm/mmap_lock.c
> > >> > +++ b/mm/mmap_lock.c
> > >> > @@ -338,6 +338,35 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
> > >> >          return NULL;
> > >> >  }
> > >> >
> > >> > +/*
> > >> > + * Find the VMA covering 'address' and lock it for reading. Waits for writers to
> > >> > + * finish if the VMA is being modified. Returns NULL if there is no VMA covering
> > >> > + * 'address'.
> > >>
> > >> Hm but it can also return NULL when vm_refcnt overflows, in theory.
> > >> Should we also return -EAGAIN (like uffd_lock_vma() below), or just retry in
> > >> here and hope for the best? The latter would be simpler for the users.
> > >> (AFAICS due to VM_REFCNT_LIMIT we never end up triggering the refcount
> > >> saturation)
> > >
> > > The problem is everything's unlocked so 'didn't find a VMA' doesn't really mean
> > > much more than 'something went wrong' because hey maybe if you check again now
> > > you'll find something :)
> >
> > Well there might be use cases where you know that either there's a vma with
> > your address and then you need to do something with it, or there's not and
> > then you don't. And it can't suddenly appear after you check.
>
> You don't hold a lock that prevents new VMAs appearing/disappearing
> spontaneously at the point you call lock_vma_under_rcu(), or after you drop the
> mmap read lock, only that at the point of checking a VMA spans address, so
> there's nothing preventing a VMA suddenly appearing after you check right? Or it
> not being the one you wanted?
>
> And checking to see if it's 'really the one you meant' is itself fraught (see
> the whole uffd saga on that).
>
> Point I'm making is that in any case where you'd actually care you'd need to
> take a stronger lock anyway, so it's actually potentially dangerous to
> differentiate between the two.

Yeah, I tend to agree with Lorenzo that  when lock_vma_under_rcu()
fails, we should not make any assumptions about the reason because the
range is not locked and therefore is not stable. Any assumption risks
being wrong if a race occurs.

>
> Given the overflow is very very unlikely I think it's also not a big deal to not
> differentiate anyway.
>
> >
> > So in that case treating that spurious NULL as "there's no vma so I don't
> > need to do anything" would be wrong.
> >
> > The usages in 4/5 and 5/5 seem like they are not this case though. So it's
> > fine. But perhaps worth just mentioning it in the comment then.
>
> Agree this is worth spelling out in the comment (I raised similarly).
>
> Maybe something like:
>
>         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.
>
>         Nothing prevents VMAs being unmapped/mapped before or after the VMA is
>         looked up, if a stronger guarantee is required, take an mmap lock.

This last statement is true only if the function returns NULL, so I
think it should be in the same paragraph as the "If no VMA is
mapped..." sentence and prepended with "In this case...". So:

       If no VMA is mapped there or, very unlikely, a reference count overflow
       occurred, return NULL. In this case, nothing prevents VMAs
being unmapped/
       mapped before or after the VMA is looked up, if a stronger guarantee is
       required, take an mmap lock.

Does that sounds good?

>
> >
> > > So I think this might be a feature more than a bug, especially given overflow is
> > > not exactly likely.
> > >
> > >>
> > >> > + *
> > >> > + * Use only in code paths where no mmap_lock and no VMA lock is held.
> > >> > + *
> > >> > + * The fast path does not take mmap_lock.
> > >> > + */
> > >> > +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)
> > >>
> > >
> > > --
> > > Cheers, Lorenzo
> >
>
> --
> Cheers, Lorenzo