mm: mremap: unlink anon_vmas when mremap with MREMAP_DONTUNMAP success

mremap with MREMAP_DONTUNMAP can move all page table entries to new vma,
which means all pages allocated for the old vma are not relevant to it
anymore, and the relevant anon_vma links needs to be unlinked, in nature
the old vma is much like been freshly created and have no pages been fault
in.

But we should not do unlink, if the new vma has effectively merged with
the old one.

[lixinhai.lxh@gmail.com: v2]
  Link: https://lkml.kernel.org/r/20210127083917.309264-2-lixinhai.lxh@gmail.com

Link: https://lkml.kernel.org/r/20210119075126.3513154-2-lixinhai.lxh@gmail.comSigned-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Lokesh Gidra <lokeshgidra@google.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mremap.c

@@ -593,6 +593,14 @@ static unsigned long move_vma(struct vm_area_struct *vma,
 		/* We always clear VM_LOCKED[ONFAULT] on the old vma */
 		vma->vm_flags &= VM_LOCKED_CLEAR_MASK;
 
+		/*
+		 * anon_vma links of the old vma is no longer needed after its page
+		 * table has been moved.
+		 */
+		if (new_vma != vma && vma->vm_start == old_addr &&
+			vma->vm_end == (old_addr + old_len))
+			unlink_anon_vmas(vma);
+
 		/* Because we won't unmap we don't need to touch locked_vm */
 		return new_addr;
 	}