On Wed, Mar 01, 2023 at 07:43:33AM +0000, Hyeonggon Yoo wrote:

On Mon, Feb 27, 2023 at 09:36:17AM -0800, Suren Baghdasaryan wrote:

quoted

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

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 mm/mmap.c  | 1 +
 mm/nommu.c | 5 +++++
 2 files changed, 6 insertions(+)

diff --git a/mm/mmap.c b/mm/mmap.c
index 1f42b9a52b9b..f7ed357056c4 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c

@@ -2255,6 +2255,7 @@ 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);

I may be missing something, but have few questions:

	1) Why does a writer need to both write-lock a VMA and mark the VMA detached
	   when unmapping it, isn't it enough to just only write-lock a VMA?

	2) as VMAs that are going to be removed are already locked in vma_prepare(),
	   so I think this hunk could be dropped?

After sending this just realized that I did not consider simple munmap case :)
But I still think 1) and 3) are valid question.

quoted

 	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);

	3) I think this hunk could be dropped as Per-VMA lock depends on MMU anyway.

Thanks,
Hyeonggon

quoted

 
 	/* 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();

-- 
2.39.2.722.g9855ee24e9-goog