Le 09/09/2022 à 02:51, Suren Baghdasaryan a écrit :

On Tue, Sep 6, 2022 at 8:35 AM Laurent Dufour [off-list ref] wrote:

quoted

Le 01/09/2022 à 19:34, Suren Baghdasaryan a écrit :

quoted

vma_adjust modifies a VMA and possibly its neighbors. Mark them as locked
before making the modifications.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 mm/mmap.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index f89c9b058105..ed58cf0689b2 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c

@@ -710,6 +710,10 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
      long adjust_next = 0;
      int remove_next = 0;

+     vma_mark_locked(vma);
+     if (next)
+             vma_mark_locked(next);
+

I was wondering if the VMAs insert and expand should be locked too.

For expand, I can't see any valid reason, but for insert, I'm puzzled.
I would think that it is better to lock the VMA to be inserted but I can't
really justify that.

It may be nice to detail why this is not need to lock insert and expand here.

'expand' is always locked before it's passed to __vma_adjust() by
vma_merge(). It has to be locked before we decide "Can it merge with
the predecessor?" here
https://elixir.bootlin.com/linux/latest/source/mm/mmap.c#L1201 because
a change in VMA can affect that decision. I spent many hours tracking
the issue caused by not locking the VMA before making this decision.
It might be good to add a comment about this...

AFAIKT 'insert' is only used by __split_vma() and it's always a brand
new VMA which is not yet linked into mm->mmap. Any reason
__vma_adjust() should lock it?

No, I think that's good this way.

quoted

quoted

      if (next && !insert) {
              struct vm_area_struct *exporter = NULL, *importer = NULL;

@@ -754,8 +758,11 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
                       * If next doesn't have anon_vma, import from vma after
                       * next, if the vma overlaps with it.
                       */
-                     if (remove_next == 2 && !next->anon_vma)
+                     if (remove_next == 2 && !next->anon_vma) {
                              exporter = next->vm_next;
+                             if (exporter)
+                                     vma_mark_locked(exporter);
+                     }

              } else if (end > next->vm_start) {
                      /*

@@ -931,6 +938,8 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
                       * "vma->vm_next" gap must be updated.
                       */
                      next = vma->vm_next;
+                     if (next)
+                             vma_mark_locked(next);
              } else {
                      /*
                       * For the scope of the comment "next" and

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