title: What is cpumask in mm_struct
body: <p>I am reading TLB shootdown code in linux kernel and I saw that shootdown ipi's were sent only to cpu's set in <code>cpu_vm_mask_var</code> in the corresponding <code>mm_struct</code> but I couldn't find where the <code>cpu_vm_mask_var</code> is being updated. </p>
  
  <p>So the questions are:</p>
  
  <ol>
  <li>What does <code>cpu_vm_mask_var</code> field in <code>mm_struct</code> represent?</li>
  <li>Where is it being updated?</li>
  </ol>
  
  <p>I think in shootdown case <code>cpu_vm_mask_var</code> should say how many CPU's contains present processes TLB entries but is that what exactly maintained by <code>cpu_vm_mask_var</code>?</p>