Re: [PATCH] mm/mmap/vma_merge: always check invariants

[Vlastimil Babka <vbabka@suse.cz>]

On 4/30/23 22:19, Lorenzo Stoakes wrote:
> We may still have inconsistent input parameters even if we choose not to
> merge and the vma_merge() invariant checks are useful for checking this
> with no production runtime cost (these are only relevant when
> CONFIG_DEBUG_VM is specified).
> 
> Therefore, perform these checks regardless of whether we merge.
> 
> This is relevant, as a recent issue (addressed in commit "mm/mempolicy:
> Correctly update prev when policy is equal on mbind") in the mbind logic
> was only picked up in the 6.2.y stable branch where these assertions are
> performed prior to determining mergeability.
> 
> Had this remained the same in mainline this issue may have been picked up
> faster, so moving forward let's always check them.
> 
> Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  mm/mmap.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 5522130ae606..13678edaa22c 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -960,17 +960,17 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  		merge_next = true;
>  	}
>  
> +	/* Verify some invariant that must be enforced by the caller. */
> +	VM_WARN_ON(prev && addr <= prev->vm_start);
> +	VM_WARN_ON(curr && (addr != curr->vm_start || end > curr->vm_end));
> +	VM_WARN_ON(addr >= end);
> +
>  	if (!merge_prev && !merge_next)
>  		return NULL; /* Not mergeable. */
>  
>  	res = vma = prev;
>  	remove = remove2 = adjust = NULL;
>  
> -	/* Verify some invariant that must be enforced by the caller. */
> -	VM_WARN_ON(prev && addr <= prev->vm_start);
> -	VM_WARN_ON(curr && (addr != curr->vm_start || end > curr->vm_end));
> -	VM_WARN_ON(addr >= end);
> -
>  	/* Can we merge both the predecessor and the successor? */
>  	if (merge_prev && merge_next &&
>  	    is_mergeable_anon_vma(prev->anon_vma, next->anon_vma, NULL)) {