Re: [PATCH v3 4/4] mm/mmap/vma_merge: init cleanup, be explicit about the non-mergeable case

[Vlastimil Babka]

On 3/22/23 08:13, Lorenzo Stoakes wrote:
> Rather than setting err = -1 and only resetting if we hit merge cases,
> explicitly check the non-mergeable case to make it abundantly clear that we
> only proceed with the rest if something is mergeable, default err to 0 and
> only update if an error might occur.
> 
> Move the merge_prev, merge_next cases closer to the logic determining curr,
> next and reorder initial variables so they are more logically grouped.
> 
> This has no functional impact.
> 
> Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>

Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>

Some nits:

> ---
>  mm/mmap.c | 57 ++++++++++++++++++++++++++++---------------------------
>  1 file changed, 29 insertions(+), 28 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 2a4f63716231..642f3d063be1 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -909,18 +909,18 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  			struct vm_userfaultfd_ctx vm_userfaultfd_ctx,
>  			struct anon_vma_name *anon_name)
>  {
> -	pgoff_t pglen = (end - addr) >> PAGE_SHIFT;
> -	pgoff_t vma_pgoff;
>  	struct vm_area_struct *curr, *next, *res;
>  	struct vm_area_struct *vma, *adjust, *remove, *remove2;
> -	int err = -1;
> +	struct vma_prepare vp;
> +	pgoff_t vma_pgoff;
> +	int err = 0;
>  	bool merge_prev = false;
>  	bool merge_next = false;
>  	bool vma_expanded = false;
> -	struct vma_prepare vp;
> +	unsigned long vma_start = addr;
>  	unsigned long vma_end = end;
> +	pgoff_t pglen = (end - addr) >> PAGE_SHIFT;
>  	long adj_start = 0;
> -	unsigned long vma_start = addr;
>  
>  	validate_mm(mm);
>  	/*
> @@ -939,36 +939,38 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  	else
>  		next = NULL;		/* case 5 */
>  
> -	/*
> -	 * By default, we return prev. Cases 3, 4, 8 will instead return next
> -	 * and cases 3, 8 will also update vma to point at next.
> -	 */
> -	res = vma = prev;
> -
> -	/* 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 (prev) {
>  		vma_start = prev->vm_start;
>  		vma_pgoff = prev->vm_pgoff;
> +
>  		/* Can we merge the predecessor? */
> -		if (prev->vm_end == addr && mpol_equal(vma_policy(prev), policy)
> +		if (addr == prev->vm_end && mpol_equal(vma_policy(prev), policy)
>  		    && can_vma_merge_after(prev, vm_flags, anon_vma, file,
> -				   pgoff, vm_userfaultfd_ctx, anon_name)) {
> +					   pgoff, vm_userfaultfd_ctx, anon_name)) {
>  			merge_prev = true;
>  			vma_prev(vmi);
>  		}
>  	}
>  
>  	/* Can we merge the successor? */
> -	if (next && mpol_equal(policy, vma_policy(next)) &&
> -	    can_vma_merge_before(next, vm_flags,
> -				 anon_vma, file, pgoff+pglen,
> -				 vm_userfaultfd_ctx, anon_name)) {
> -		merge_next = true;
> -	}
> +	merge_next = next && mpol_equal(policy, vma_policy(next)) &&
> +		can_vma_merge_before(next, vm_flags,
> +				     anon_vma, file, pgoff+pglen,
> +				     vm_userfaultfd_ctx, anon_name);

Not a great fan of this, I think the if() is more readable, but if you and
Liam agree, I won't mind much. Either way could consolidate the parameters
on less lines maybe.

> +
> +	if (!merge_prev && !merge_next)
> +		return NULL; /* Not mergeable. */
> +
> +	/*
> +	 * By default, we return prev. Cases 3, 4, 8 will instead return next
> +	 * and cases 3, 8 will also update vma to point at next.
> +	 */
> +	res = vma = prev;
> +
> +	/* 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);
>  
>  	remove = remove2 = adjust = NULL;
>  	/* Can we merge both the predecessor and the successor? */
> @@ -984,7 +986,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  				err = dup_anon_vma(prev, curr);
>  		}
>  	} else if (merge_prev) {
> -		err = 0;				/* case 2 */
> +							/* case 2 */

Move the comment from this now weirdly empty line to the "else if" one above?

>  		if (curr) {
>  			err = dup_anon_vma(prev, curr);
>  			if (end == curr->vm_end) {	/* case 7 */
> @@ -994,7 +996,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  				adj_start = (end - curr->vm_start);
>  			}
>  		}
> -	} else if (merge_next) {
> +	} else { /* merge_next */
>  		res = next;
>  		if (prev && addr < prev->vm_end) {	/* case 4 */
>  			vma_end = addr;
> @@ -1010,7 +1012,6 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  			vma_start = addr;
>  			vma_end = next->vm_end;
>  			vma_pgoff = next->vm_pgoff;
> -			err = 0;
>  			if (curr) {			/* case 8 */
>  				vma_pgoff = curr->vm_pgoff;
>  				remove = curr;
> @@ -1019,7 +1020,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  		}
>  	}
>  
> -	/* Cannot merge or error in anon_vma clone */
> +	/* Error in anon_vma clone. */
>  	if (err)
>  		return NULL;
>