Re: [PATCH v3 1/5] mm: introduce vma_start_read_locked{_nested} helpers

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Sun, Nov 17, 2024 at 12:09:27AM -0800, Suren Baghdasaryan wrote:
> Introduce helper functions which can be used to read-lock a VMA when
> holding mmap_lock for read. Replace direct accesses to vma->vm_lock
> with these new helpers.
>
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>

LGTM thanks:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> ---
>  include/linux/mm.h | 24 ++++++++++++++++++++++++
>  mm/userfaultfd.c   | 22 +++++-----------------
>  2 files changed, 29 insertions(+), 17 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index fecd47239fa9..1ba2e480ae63 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -722,6 +722,30 @@ static inline bool vma_start_read(struct vm_area_struct *vma)
>  	return true;
>  }
>
> +/*
> + * 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
> + * 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.
> + */
> +static inline void vma_start_read_locked_nested(struct vm_area_struct *vma, int subclass)
> +{
> +	mmap_assert_locked(vma->vm_mm);
> +	down_read_nested(&vma->vm_lock->lock, subclass);
> +}
> +
> +/*
> + * 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
> + * 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.
> + */
> +static inline void vma_start_read_locked(struct vm_area_struct *vma)
> +{
> +	mmap_assert_locked(vma->vm_mm);
> +	down_read(&vma->vm_lock->lock);
> +}
> +
>  static inline void vma_end_read(struct vm_area_struct *vma)
>  {
>  	rcu_read_lock(); /* keeps vma alive till the end of up_read */
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 60a0be33766f..87db4b32b82a 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -84,16 +84,8 @@ static struct vm_area_struct *uffd_lock_vma(struct mm_struct *mm,
>
>  	mmap_read_lock(mm);
>  	vma = find_vma_and_prepare_anon(mm, address);
> -	if (!IS_ERR(vma)) {
> -		/*
> -		 * We cannot use vma_start_read() as it may fail due to
> -		 * false locked (see comment in vma_start_read()). We
> -		 * can avoid that by directly locking vm_lock under
> -		 * mmap_lock, which guarantees that nobody can lock the
> -		 * vma for write (vma_start_write()) under us.
> -		 */
> -		down_read(&vma->vm_lock->lock);
> -	}
> +	if (!IS_ERR(vma))
> +		vma_start_read_locked(vma);
>
>  	mmap_read_unlock(mm);
>  	return vma;
> @@ -1476,14 +1468,10 @@ static int uffd_move_lock(struct mm_struct *mm,
>  	mmap_read_lock(mm);
>  	err = find_vmas_mm_locked(mm, dst_start, src_start, dst_vmap, src_vmap);
>  	if (!err) {
> -		/*
> -		 * See comment in uffd_lock_vma() as to why not using
> -		 * vma_start_read() here.
> -		 */
> -		down_read(&(*dst_vmap)->vm_lock->lock);
> +		vma_start_read_locked(*dst_vmap);
>  		if (*dst_vmap != *src_vmap)
> -			down_read_nested(&(*src_vmap)->vm_lock->lock,
> -					 SINGLE_DEPTH_NESTING);
> +			vma_start_read_locked_nested(*src_vmap,
> +						SINGLE_DEPTH_NESTING);
>  	}
>  	mmap_read_unlock(mm);
>  	return err;
> --
> 2.47.0.338.g60cca15819-goog
>