Re: [PATCH] mm/vma: predicate vma_assert_stabilised() on CONFIG_DEBUG_VM

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 11/10/26 16:10, Lorenzo Stoakes (ARM) wrote:
> I introduced commit 17fd82c3abe0 ("mm/vma: add and use
> vma_assert_stabilised()") with the intent of picking up programmatic bugs
> when debugging mm code, but kept the general practice of asserting even
> when CONFIG_DEBUG_VM is not set in line with existing code.
> 
> However, it turns out doing so can cause performance regressions,
> especially when asserted in vma_has_anon_rmap() which is referenced in a
> number of hotpaths.
> 
> Since the intent was always a debug-only assertion, explicitly check for
> CONFIG_DEBUG_VM and do not assert if it isn't set.
> 
> Really all of the assert paths need to be predicated on this, but start
> with this one only, out of caution.
> 
> Additionally, since a detached VMA is trivially stabilised (nothing to
> stabilise against), check for this case at the outset, and update
> vma_has_anon_rmap() to not explicitly need to check for this.
> 
> This also, importantly, avoids reading any VMA fields if CONFIG_DEBUG_VM is
> not set.
> 
> Target the commit which introduced vma_has_anon_rmap() for the fix, as this
> is where the issue started manifesting in practice.
> 
> Fixes: 38564710f443 ("mm: implement and use vma_has_anon_rmap(), silence KCSAN")
> Reported-by: Seunghyun An <san55@wisc.edu>
> Closes: https://lore.kernel.org/linux-mm/20261110000002.1834454-1-san55@wisc.edu/
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

> ---
> Note that vma_is_anon_rmap() was introduced in 7.4, so no need for a cc: stable,
> just needs to be sent to Linus as a hotfix. Thanks!
> ---
>  include/linux/mmap_lock.h | 19 ++++++++++++++-----
>  mm/vma.h                  |  3 +--
>  2 files changed, 15 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
> index 9ccbbcbd7aca..9c3a9c6525c1 100644
> --- a/include/linux/mmap_lock.h
> +++ b/include/linux/mmap_lock.h
> @@ -408,6 +408,11 @@ static inline void vma_assert_locked(const struct vm_area_struct *vma)
>  	vma_assert_write_locked(vma);
>  }
>  
> +static inline bool vma_is_attached(const struct vm_area_struct *vma)
> +{
> +	return refcount_read(&vma->vm_refcnt);
> +}
> +
>  /**
>   * vma_assert_stabilised() - assert that this VMA cannot be changed from
>   * underneath us either by having a VMA or mmap lock held.
> @@ -418,9 +423,18 @@ static inline void vma_assert_locked(const struct vm_area_struct *vma)
>   *
>   * With lockdep disabled we may sometimes race with other threads acquiring the
>   * mmap read lock simultaneous with our VMA read lock.
> + *
> + * Assert only if CONFIG_DEBUG_VM is set.
>   */
>  static inline void vma_assert_stabilised(const struct vm_area_struct *vma)
>  {
> +	if (!IS_ENABLED(CONFIG_DEBUG_VM))
> +		return;
> +
> +	/* Detached VMAs are trivially stabilised. */
> +	if (!vma_is_attached(vma))
> +		return;
> +
>  	/*
>  	 * If another thread owns an mmap lock, it may go away at any time, and
>  	 * thus is no guarantee of stability.
> @@ -454,11 +468,6 @@ static inline void vma_assert_stabilised(const struct vm_area_struct *vma)
>  	vma_assert_locked(vma);
>  }
>  
> -static inline bool vma_is_attached(const struct vm_area_struct *vma)
> -{
> -	return refcount_read(&vma->vm_refcnt);
> -}
> -
>  /*
>   * WARNING: to avoid racing with vma_mark_attached()/vma_mark_detached(), these
>   * assertions should be made either under mmap_write_lock or when the object
> diff --git a/mm/vma.h b/mm/vma.h
> index 3fc3b47a954b..05300badd70b 100644
> --- a/mm/vma.h
> +++ b/mm/vma.h
> @@ -278,8 +278,7 @@ static inline pgoff_t vmg_end_pgoff(const struct vma_merge_struct *vmg)
>   */
>  static inline bool vma_has_anon_rmap(const struct vm_area_struct *vma)
>  {
> -	if (vma_is_attached(vma))
> -		vma_assert_stabilised(vma);
> +	vma_assert_stabilised(vma);
>  	/* KCSAN gets confused about the optimistic check. Silence it. */
>  	return data_race(vma->anon_vma);
>  }
> 
> ---
> base-commit: 93d7a816a2df3e907e2719bd12b0073f1285fc5c
> change-id: 20261110-vma-fixup-debug-asserts-2d327fe27f8c
> 
> Cheers,