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

[Wei Yang <richard.weiyang@xxxxxxxxx>]

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@xxxxxxxxx>
CC: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>

---
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