mm/vma: update vmg_adjust_set_range() to offset pgoff instead

We are calculating the pgoff as an offset, since we have vma_add_pgoff()
and vma_sub_pgoff() available, just offset this value directly and use
__vma_set_range() for vma->vm_[start, end] values.

We take care to update the range before offsetting the page offset, so the
adjusted VMA's vm_start and vm_pgoff are mutually consistent at the point
the page offset helpers operate - this matters once vma_set_pgoff() comes
to assert invariants which relate the two.

Doing so lays the foundation for future work which allows for use of
virtual page offsets for MAP_PRIVATE-file backed mappings.

No functional change intended.

Link: https://lore.kernel.org/20260710-b4-pre-scalable-cow-v2-27-2a5aa403d977@kernel.org
Signed-off-by: Lorenzo Stoakes <ljs@kernel.org>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Reviewed-by: Gregory Price <gourry@gourry.net>
Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Cc: Ackerley Tng <ackerleytng@google.com>
Cc: David Hildenbrand (Arm) <david@kernel.org>
Cc: Kai Huang <kai.huang@intel.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: SJ Park <sj@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Liam R. Howlett (Oracle) <liam@infradead.org>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 4 insertions, 11 deletions

diff --git a/mm/vma.c b/mm/vma.c
index c75322d0ffc2..78af0e530066 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -714,9 +714,6 @@ void validate_mm(struct mm_struct *mm)
  */
 static void vmg_adjust_set_range(struct vma_merge_struct *vmg)
 {
-	struct vm_area_struct *adjust;
-	pgoff_t pgoff;
-
 	if (vmg->__adjust_middle_start) {
 		/*
 		 * vmg->start    vmg->end
@@ -735,8 +732,8 @@ static void vmg_adjust_set_range(struct vma_merge_struct *vmg)
 		struct vm_area_struct *middle = vmg->middle;
 		const unsigned long delta = vmg->end - middle->vm_start;
 
-		pgoff = vma_start_pgoff(middle) + (delta >> PAGE_SHIFT);
-		adjust = middle;
+		__vma_set_range(middle, vmg->end, middle->vm_end);
+		vma_add_pgoff(middle, delta >> PAGE_SHIFT);
 	} else if (vmg->__adjust_next_start) {
 		/*
 		 *                Originally:
@@ -764,13 +761,9 @@ static void vmg_adjust_set_range(struct vma_merge_struct *vmg)
 		struct vm_area_struct *next = vmg->next;
 		const unsigned long delta = next->vm_start - vmg->end;
 
-		pgoff = vma_start_pgoff(next) - (delta >> PAGE_SHIFT);
-		adjust = next;
-	} else {
-		return;
+		__vma_set_range(next, vmg->end, next->vm_end);
+		vma_sub_pgoff(next, delta >> PAGE_SHIFT);
 	}
-
-	vma_set_range(adjust, vmg->end, adjust->vm_end, pgoff);
 }
 
 /*