Re: [PATCH v2 2/6] mm/munmap: Replace can_modify_mm with can_modify_vma

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* Pedro Falcato <pedro.falcato@gmail.com> [240807 17:13]:
> We were doing an extra mmap tree traversal just to check if the entire
> range is modifiable. This can be done when we iterate through the VMAs
> instead.
> 
> Signed-off-by: Pedro Falcato <pedro.falcato@gmail.com>
> ---
>  mm/mmap.c | 13 +------------
>  mm/vma.c  | 23 ++++++++++++-----------
>  2 files changed, 13 insertions(+), 23 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 4a9c2329b09..c1c7a7d00f5 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1740,18 +1740,7 @@ int do_vma_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  		unsigned long start, unsigned long end, struct list_head *uf,
>  		bool unlock)
>  {
> -	struct mm_struct *mm = vma->vm_mm;
> -
> -	/*
> -	 * Check if memory is sealed before arch_unmap.
> -	 * Prevent unmapping a sealed VMA.
> -	 * can_modify_mm assumes we have acquired the lock on MM.
> -	 */
> -	if (unlikely(!can_modify_mm(mm, start, end)))
> -		return -EPERM;
> -
> -	arch_unmap(mm, start, end);
> -	return do_vmi_align_munmap(vmi, vma, mm, start, end, uf, unlock);
> +	return do_vmi_align_munmap(vmi, vma, vma->vm_mm, start, end, uf, unlock);
>  }
>  
>  /*
> diff --git a/mm/vma.c b/mm/vma.c
> index bf0546fe6ea..7a121bcc907 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -712,6 +712,12 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  		if (end < vma->vm_end && mm->map_count >= sysctl_max_map_count)
>  			goto map_count_exceeded;
>  
> +		/* Don't bother splitting the VMA if we can't unmap it anyway */
> +		if (!can_modify_vma(vma)) {
> +			error = -EPERM;
> +			goto start_split_failed;
> +		}
> +

Would this check be better placed in __split_vma()?  It could replace
both this and the next chunk of code.

>  		error = __split_vma(vmi, vma, start, 1);
>  		if (error)
>  			goto start_split_failed;
> @@ -723,6 +729,11 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  	 */
>  	next = vma;
>  	do {
> +		if (!can_modify_vma(vma)) {
> +			error = -EPERM;
> +			goto modify_vma_failed;
> +		}
> +
>  		/* Does it split the end? */
>  		if (next->vm_end > end) {
>  			error = __split_vma(vmi, next, end, 0);
> @@ -815,6 +826,7 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  	__mt_destroy(&mt_detach);
>  	return 0;
>  
> +modify_vma_failed:
>  clear_tree_failed:
>  userfaultfd_error:
>  munmap_gather_failed:
> @@ -860,17 +872,6 @@ int do_vmi_munmap(struct vma_iterator *vmi, struct mm_struct *mm,
>  	if (end == start)
>  		return -EINVAL;
>  
> -	/*
> -	 * Check if memory is sealed before arch_unmap.
> -	 * Prevent unmapping a sealed VMA.
> -	 * can_modify_mm assumes we have acquired the lock on MM.
> -	 */
> -	if (unlikely(!can_modify_mm(mm, start, end)))
> -		return -EPERM;
> -
> -	 /* arch_unmap() might do unmaps itself.  */
> -	arch_unmap(mm, start, end);
> -
>  	/* Find the first overlapping VMA */
>  	vma = vma_find(vmi, end);
>  	if (!vma) {
> -- 
> 2.46.0
>