Subject: [PATCH v2 18/33] mm: write-lock VMAs before removing them from VMA tree

From: Suren Baghdasaryan <[email-masked]>
Date: Fri, 27 Jan 2023 11:40:55 -0800

Write-locking VMAs before isolating them ensures that page fault
handlers don't operate on isolated VMAs.

Signed-off-by: Suren Baghdasaryan <[email-masked]>
---
 mm/mmap.c  | 1 +
 mm/nommu.c | 5 +++++
 2 files changed, 6 insertions(+)

diff --git a/mm/mmap.c b/mm/mmap.c
index b3c247073aa0..5bdfd087b632 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2261,6 +2261,7 @@ int split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
 static inline int munmap_sidetree(struct vm_area_struct *vma,
 				   struct ma_state *mas_detach)
 {
+	vma_start_write(vma);
 	mas_set_range(mas_detach, vma->vm_start, vma->vm_end - 1);
 	if (mas_store_gfp(mas_detach, vma, GFP_KERNEL))
 		return -ENOMEM;
diff --git a/mm/nommu.c b/mm/nommu.c
index 57ba243c6a37..2ab162d773e2 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -588,6 +588,7 @@ static int delete_vma_from_mm(struct vm_area_struct *vma)
 		       current->pid);
 		return -ENOMEM;
 	}
+	vma_start_write(vma);
 	cleanup_vma_from_mm(vma);
 
 	/* remove from the MM's tree and list */
@@ -1519,6 +1520,10 @@ void exit_mmap(struct mm_struct *mm)
 	 */
 	mmap_write_lock(mm);
 	for_each_vma(vmi, vma) {
+		/*
+		 * No need to lock VMA because this is the only mm user and no
+		 * page fault handled can race with it.
+		 */
 		cleanup_vma_from_mm(vma);
 		delete_vma(mm, vma);
 		cond_resched();
-- 
2.39.1