Re: [PATCH 1/3] mmap: Fix vma_iterator in error path of vma_merge()

[Vlastimil Babka <vbabka@suse.cz>]

On 9/27/23 18:07, Liam R. Howlett wrote:
> When merging of the previous VMA fails after the vma iterator has been
> moved to the previous entry, the vma iterator must be advanced to ensure
> the caller takes the correct action on the next vma iterator event.  Fix
> this by adding a vma_next() call to the error path.
> 
> Users may experience higher CPU usage, most likely in very low memory
> situations.

Maybe we could say explicitly that before this fix, vma_merge will be called
twice on the same vma, which to the best of our knowledge does not cause
anything worse than some wasted cycles because vma == prev, but it's fragile?

> Link: https://lore.kernel.org/linux-mm/CAG48ez12VN1JAOtTNMY+Y2YnsU45yL5giS-Qn=ejtiHpgJAbdQ@mail.gmail.com/
> Closes: https://lore.kernel.org/linux-mm/CAG48ez12VN1JAOtTNMY+Y2YnsU45yL5giS-Qn=ejtiHpgJAbdQ@mail.gmail.com/
> Fixes: 18b098af2890 ("vma_merge: set vma iterator to correct position.")
> Cc: stable@vger.kernel.org
> Cc: Jann Horn <jannh@google.com>
> Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
> ---
>  mm/mmap.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index b56a7f0c9f85..b5bc4ca9bdc4 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -968,14 +968,14 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  				vma_pgoff = curr->vm_pgoff;
>  				vma_start_write(curr);
>  				remove = curr;
> -				err = dup_anon_vma(next, curr);
> +				err = dup_anon_vma(next, curr, &anon_dup);
>  			}
>  		}
>  	}
>  
>  	/* Error in anon_vma clone. */
>  	if (err)
> -		return NULL;
> +		goto anon_vma_fail;
>  
>  	if (vma_start < vma->vm_start || vma_end > vma->vm_end)
>  		vma_expanded = true;

The vma_iter_config() actions done in this part are something we don't need
to undo?

> @@ -988,7 +988,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  	}
>  
>  	if (vma_iter_prealloc(vmi, vma))
> -		return NULL;
> +		goto prealloc_fail;



>  	init_multi_vma_prep(&vp, vma, adjust, remove, remove2);
>  	VM_WARN_ON(vp.anon_vma && adjust && adjust->anon_vma &&
> @@ -1016,6 +1016,12 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  	vma_complete(&vp, vmi, mm);
>  	khugepaged_enter_vma(res, vm_flags);
>  	return res;
> +
> +prealloc_fail:
> +anon_vma_fail:
> +	if (merge_prev)
> +		vma_next(vmi);
> +	return NULL;
>  }
>  
>  /*