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Date:   Mon,  9 Jan 2023 12:53:27 -0800
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Subject: [PATCH 32/41] mm: prevent userfaults to be handled under per-vma lock
From:   Suren Baghdasaryan <surenb@google.com>
To:     akpm@linux-foundation.org
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Precedence: bulk

Due to the possibility of handle_userfault dropping mmap_lock, avoid fault
handling under VMA lock and retry holding mmap_lock. This can be handled
more gracefully in the future.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Suggested-by: Peter Xu <peterx@redhat.com>
---
 mm/memory.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/mm/memory.c b/mm/memory.c
index 20806bc8b4eb..12508f4d845a 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5273,6 +5273,13 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
 	if (!vma->anon_vma)
 		goto inval;
 
+	/*
+	 * Due to the possibility of userfault handler dropping mmap_lock, avoid
+	 * it for now and fall back to page fault handling under mmap_lock.
+	 */
+	if (userfaultfd_armed(vma))
+		goto inval;
+
 	if (!vma_read_trylock(vma))
 		goto inval;
 
-- 
2.39.0