[PATCH v2] mm/vma: the pgoff is correct if can_merge_right

[PATCH v2] mm/vma: the pgoff is correct if can_merge_right

[Wei Yang]

By this point can_vma_merge_right() must have returned true, which implies
can_vma_merge_before() also returned true, which already asserts that the
pgoff is as expected for a merge with the following VMA, thus this
assignment is redundant.

Below is a more detail explanation.

Current definition of can_vma_merge_right() is:

	static bool can_vma_merge_right(struct vma_merge_struct *vmg,
					bool can_merge_left)
	{
		if (!vmg->next || vmg->end != vmg->next->vm_start ||
		    !can_vma_merge_before(vmg))
			return false;
		...
	}

And:

	static bool can_vma_merge_before(struct vma_merge_struct *vmg)
	{
		pgoff_t pglen = PHYS_PFN(vmg->end - vmg->start);
	...
			if (vmg->next->vm_pgoff == vmg->pgoff + pglen)
				return true;
	...
	}

Which implies vmg->pgoff == vmg->next->vm_pgoff - pglen.

None of these values are changed between the check and prior assignment,
so this was an entirely redundant assignment.

[lorenzo: rephrase the change log]

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
CC: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

---
v2: rephrase the change log per Lorenzo's suggestion.
---
 mm/vma.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/mm/vma.c b/mm/vma.c
index 4737afcb064c..fb4f1863f88e 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -915,7 +915,6 @@ struct vm_area_struct *vma_merge_new_range(struct vma_merge_struct *vmg)
 	unsigned long start = vmg->start;
 	unsigned long end = vmg->end;
 	pgoff_t pgoff = vmg->pgoff;
-	pgoff_t pglen = PHYS_PFN(end - start);
 	bool can_merge_left, can_merge_right;
 
 	mmap_assert_write_locked(vmg->mm);
@@ -936,7 +935,6 @@ struct vm_area_struct *vma_merge_new_range(struct vma_merge_struct *vmg)
 	if (can_merge_right) {
 		vmg->end = next->vm_end;
 		vmg->vma = next;
-		vmg->pgoff = next->vm_pgoff - pglen;
 	}
 
 	/* If we can merge with the previous VMA, adjust vmg accordingly. */
-- 
2.34.1

Re: [PATCH v2] mm/vma: the pgoff is correct if can_merge_right

[Lorenzo Stoakes]

On Thu, Oct 24, 2024 at 09:33:47AM +0000, Wei Yang wrote:
> By this point can_vma_merge_right() must have returned true, which implies
> can_vma_merge_before() also returned true, which already asserts that the
> pgoff is as expected for a merge with the following VMA, thus this
> assignment is redundant.
>
> Below is a more detail explanation.
>
> Current definition of can_vma_merge_right() is:
>
> 	static bool can_vma_merge_right(struct vma_merge_struct *vmg,
> 					bool can_merge_left)
> 	{
> 		if (!vmg->next || vmg->end != vmg->next->vm_start ||
> 		    !can_vma_merge_before(vmg))
> 			return false;
> 		...
> 	}
>
> And:
>
> 	static bool can_vma_merge_before(struct vma_merge_struct *vmg)
> 	{
> 		pgoff_t pglen = PHYS_PFN(vmg->end - vmg->start);
> 	...
> 			if (vmg->next->vm_pgoff == vmg->pgoff + pglen)
> 				return true;
> 	...
> 	}
>
> Which implies vmg->pgoff == vmg->next->vm_pgoff - pglen.
>
> None of these values are changed between the check and prior assignment,
> so this was an entirely redundant assignment.
>
> [lorenzo: rephrase the change log]

Thanks for this, LGTM!

>
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> CC: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>
> ---
> v2: rephrase the change log per Lorenzo's suggestion.
> ---
>  mm/vma.c | 2 --
>  1 file changed, 2 deletions(-)
>
> diff --git a/mm/vma.c b/mm/vma.c
> index 4737afcb064c..fb4f1863f88e 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -915,7 +915,6 @@ struct vm_area_struct *vma_merge_new_range(struct vma_merge_struct *vmg)
>  	unsigned long start = vmg->start;
>  	unsigned long end = vmg->end;
>  	pgoff_t pgoff = vmg->pgoff;
> -	pgoff_t pglen = PHYS_PFN(end - start);
>  	bool can_merge_left, can_merge_right;
>
>  	mmap_assert_write_locked(vmg->mm);
> @@ -936,7 +935,6 @@ struct vm_area_struct *vma_merge_new_range(struct vma_merge_struct *vmg)
>  	if (can_merge_right) {
>  		vmg->end = next->vm_end;
>  		vmg->vma = next;
> -		vmg->pgoff = next->vm_pgoff - pglen;
>  	}
>
>  	/* If we can merge with the previous VMA, adjust vmg accordingly. */
> --
> 2.34.1
>
>