Add helpers for adding or subtracting to a VMA's page offset, exposed
internally for VMA users within mm in mm/vma.h.

This is to lay the foundations for tracking anonymous page offset for
MAP_PRIVATE file-backed mappings, where adding and subtracting from this
value must be reflected in both the file and anonymous offsets.

These are used on VMA split and downward stack expansion.

No functional change intended.

Signed-off-by: Lorenzo Stoakes <ljs@kernel.org>
---
 mm/nommu.c                      |  6 ++++--
 mm/vma.c                        |  6 +++---
 mm/vma.h                        | 12 ++++++++++++
 tools/testing/vma/include/dup.h | 13 ++++++++++++-
 4 files changed, 31 insertions(+), 6 deletions(-)

diff --git a/mm/nommu.c b/mm/nommu.c
index 7333d855e974..c7fafcd87c14 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -41,6 +41,7 @@
 #include <asm/tlbflush.h>
 #include <asm/mmu_context.h>
 #include "internal.h"
+#include "vma.h"
 
 unsigned long highest_memmap_pfn;
 int heap_stack_gap = 0;
@@ -1338,7 +1339,8 @@ static int split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
 		region->vm_top = region->vm_end = new->vm_end = addr;
 	} else {
 		region->vm_start = new->vm_start = addr;
-		region->vm_pgoff = new->vm_pgoff += npages;
+		vma_add_pgoff(new, npages);
+		region->vm_pgoff = vma_start_pgoff(new);
 	}
 
 	vma_iter_config(vmi, new->vm_start, new->vm_end);
@@ -1355,7 +1357,7 @@ static int split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
 	delete_nommu_region(vma->vm_region);
 	if (new_below) {
 		vma->vm_region->vm_start = vma->vm_start = addr;
-		vma->vm_pgoff += npages;
+		vma_add_pgoff(vma, npages);
 		vma->vm_region->vm_pgoff = vma_start_pgoff(vma);
 	} else {
 		vma->vm_region->vm_end = vma->vm_end = addr;
diff --git a/mm/vma.c b/mm/vma.c
index 185d07397ca6..cb7222e20c93 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -517,7 +517,7 @@ __split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
 		new->vm_end = addr;
 	} else {
 		new->vm_start = addr;
-		new->vm_pgoff += linear_page_delta(vma, addr);
+		vma_add_pgoff(new, linear_page_delta(vma, addr));
 	}
 
 	err = -ENOMEM;
@@ -556,7 +556,7 @@ __split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
 
 	if (new_below) {
 		vma->vm_start = addr;
-		vma->vm_pgoff += (addr - new->vm_start) >> PAGE_SHIFT;
+		vma_add_pgoff(vma, (addr - new->vm_start) >> PAGE_SHIFT);
 	} else {
 		vma->vm_end = addr;
 	}
@@ -3305,7 +3305,7 @@ int expand_downwards(struct vm_area_struct *vma, unsigned long address)
 				vm_stat_account(mm, vma->vm_flags, grow);
 				anon_vma_interval_tree_pre_update_vma(vma);
 				vma->vm_start = address;
-				vma->vm_pgoff -= grow;
+				vma_sub_pgoff(vma, grow);
 				/* Overwrite old entry in mtree. */
 				vma_iter_store_overwrite(&vmi, vma);
 				anon_vma_interval_tree_post_update_vma(vma);
diff --git a/mm/vma.h b/mm/vma.h
index 2342516ce00e..47fe35e5307e 100644
--- a/mm/vma.h
+++ b/mm/vma.h
@@ -247,6 +247,18 @@ static inline pgoff_t vmg_end_pgoff(const struct vma_merge_struct *vmg)
 	return vmg_start_pgoff(vmg) + vmg_pages(vmg);
 }
 
+static inline void vma_add_pgoff(struct vm_area_struct *vma, pgoff_t delta)
+{
+	vma_assert_can_modify(vma);
+	vma->vm_pgoff += delta;
+}
+
+static inline void vma_sub_pgoff(struct vm_area_struct *vma, pgoff_t delta)
+{
+	vma_assert_can_modify(vma);
+	vma->vm_pgoff -= delta;
+}
+
 #define VMG_STATE(name, mm_, vmi_, start_, end_, vma_flags_, pgoff_)	\
 	struct vma_merge_struct name = {				\
 		.mm = mm_,						\
diff --git a/tools/testing/vma/include/dup.h b/tools/testing/vma/include/dup.h
index 7ed165c8d9bc..41fea90a344d 100644
--- a/tools/testing/vma/include/dup.h
+++ b/tools/testing/vma/include/dup.h
@@ -1163,6 +1163,11 @@ static inline struct vm_area_struct *vma_next(struct vma_iterator *vmi)
 	return mas_find(&vmi->mas, ULONG_MAX);
 }
 
+static inline bool vma_is_attached(struct vm_area_struct *vma)
+{
+	return refcount_read(&vma->vm_refcnt);
+}
+
 /*
  * WARNING: to avoid racing with vma_mark_attached()/vma_mark_detached(), these
  * assertions should be made either under mmap_write_lock or when the object
@@ -1170,7 +1175,13 @@ static inline struct vm_area_struct *vma_next(struct vma_iterator *vmi)
  */
 static inline void vma_assert_attached(struct vm_area_struct *vma)
 {
-	WARN_ON_ONCE(!refcount_read(&vma->vm_refcnt));
+	WARN_ON_ONCE(!vma_is_attached(vma));
+}
+
+static inline void vma_assert_can_modify(struct vm_area_struct *vma)
+{
+	if (vma_is_attached(vma))
+		vma_assert_write_locked(vma);
 }
 
 static inline void vma_assert_detached(struct vm_area_struct *vma)
-- 
2.54.0