mm/vma: the pgoff is correct if can_merge_right

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.

[akpm@linux-foundation.org: remove now-unused local]
[lorenzo.stoakes@oracle.com: rephrase the changelog]
Link: https://lkml.kernel.org/r/20241024093347.18057-1-richard.weiyang@gmail.com
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Jann Horn <jannh@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/vma.c b/mm/vma.c
index c26bbc898f853cb0cef9e9e2308294e38a489d93..68138e8c153e67df2baf3ac7315354578339dece 100644 (file)
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -962,9 +962,7 @@ struct vm_area_struct *vma_merge_new_range(struct vma_merge_struct *vmg)
 {
        struct vm_area_struct *prev = vmg->prev;
        struct vm_area_struct *next = vmg->next;
-       unsigned long start = vmg->start;
        unsigned long end = vmg->end;
-       pgoff_t pglen = PHYS_PFN(end - start);
        bool can_merge_left, can_merge_right;
        bool just_expand = vmg->merge_flags & VMG_FLAG_JUST_EXPAND;
 
@@ -986,7 +984,6 @@ 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. */