[PATCH v3 02/40] mm/vma: predicate setting mmap_prepare VMA fields on new vma alloc

[Lorenzo Stoakes (ARM)]

It only makes sense to manipulate VMA fields if we allocated a new VMA,
rather than merged it.

VMA merging does not compare vm_ops or vm_private_data, so a merged VMA
keeps its own, which is also what the legacy f_op->mmap path does since it
never touches an existing VMA. Previously set_vma_user_defined_fields()
overwrote the merged VMA's fields with those set for the new mapping. In
practice these are the same values, with rare exceptions such as shmem
selecting vm_ops based on whether the file has been unlinked, so no
user-visible change is expected.

Make this dependency explicit, and additionally constify have_mmap_prepare
while we're here.

Signed-off-by: Lorenzo Stoakes (ARM) <ljs at kernel.org>
---
 mm/vma.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/vma.c b/mm/vma.c
index fa784f069da4..4b74b48c29b7 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -2894,7 +2894,7 @@ static unsigned long __mmap_region(struct file *file, unsigned long addr,
 {
 	struct mm_struct *mm = current->mm;
 	struct vm_area_struct *vma = NULL;
-	bool have_mmap_prepare = file && file->f_op->mmap_prepare;
+	const bool have_mmap_prepare = file && file->f_op->mmap_prepare;
 	VMA_ITERATOR(vmi, mm, addr);
 	const pgoff_t anon_pgoff = addr >> PAGE_SHIFT;
 	MMAP_STATE(map, mm, &vmi, addr, len, pgoff, anon_pgoff, vma_flags, file);
@@ -2937,7 +2937,7 @@ static unsigned long __mmap_region(struct file *file, unsigned long addr,
 		allocated_new = true;
 	}
 
-	if (have_mmap_prepare && !map_is_anon(&map))
+	if (have_mmap_prepare && allocated_new && !map_is_anon(&map))
 		set_vma_user_defined_fields(vma, &map);
 
 	__mmap_complete(&map, vma);

-- 
2.55.0