mm/vma: add and use vma_[add/sub]_pgoff()

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.

[akpm@linux-foundation.org: use linear_page_delta() in __split_vma(), per Vlastimal & Lorenzo]
Link: https://lore.kernel.org/20260710-b4-pre-scalable-cow-v2-23-2a5aa403d977@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>

3 files changed, 19 insertions, 5 deletions

diff --git a/mm/nommu.c b/mm/nommu.c
index 81caf5318aaf..dc64834e6834 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;
@@ -1361,7 +1362,8 @@ 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);
@@ -1378,7 +1380,7 @@ 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 7aa0149f076c..6e62642e00a2 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, linear_page_delta(new, addr));
 	} 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_rmap_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_rmap_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_,						\