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

On 8/2/26 23:54, Suren Baghdasaryan wrote:
> From: Dave Hansen <[email protected]>
> 
> == 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()?

> 
>  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 <[email protected]>
> Signed-off-by: Suren Baghdasaryan <[email protected]>
> Cc: Suren Baghdasaryan <[email protected]>
> Cc: Andrew Morton <[email protected]>
> Cc: "Liam R. Howlett" <[email protected]>
> Cc: Lorenzo Stoakes <[email protected]>
> Cc: Vlastimil Babka <[email protected]>
> Cc: Shakeel Butt <[email protected]>
> Cc: [email protected]
> Cc: Greg Kroah-Hartman <[email protected]>
> Cc: Arve Hjønnevåg <[email protected]>
> Cc: Todd Kjos <[email protected]>
> Cc: Christian Brauner <[email protected]>
> Cc: Carlos Llamas <[email protected]>
> Cc: Alice Ryhl <[email protected]>
> Cc: "David S. Miller" <[email protected]>
> Cc: David Ahern <[email protected]>
> Cc: [email protected]
> ---
>  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)

> + *
> + * 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)