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I found the root cause of this lockdep warning.

In mmap_region(), unmap_region() may be called while vma_link() has not been
called. This happens during the error path if call_mmap() failed.

The only to fix that particular case is to call
seqcount_init(&vma->vm_sequence) when initializing the vma in mmap_region().

Ack, although that would require a fixup to dup_mmap() as well.

You're right, I'll fix that too.

I also think the following is needed:

diff --git a/fs/exec.c b/fs/exec.c
--- a/fs/exec.c
+++ b/fs/exec.c

@@ -312,6 +312,10 @@ static int __bprm_mm_init(struct linux_binprm *bprm)
 	vma->vm_flags = VM_SOFTDIRTY | VM_STACK_FLAGS | VM_STACK_INCOMPLETE_SETUP;
 	vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
 	INIT_LIST_HEAD(&vma->anon_vma_chain);
+#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
+	seqcount_init(&vma->vm_sequence);
+	atomic_set(&vma->vm_ref_count, 0);
+#endif
 
 	err = insert_vm_struct(mm, vma);
 	if (err)