Overflow detection in unsigned division algorithm

I have a question about 64/32-bits division algorithm as it appears in Hacker's Delight in Chapter 9-4 Unsigned Long Division, Figure 9-3, "div1u". Online it can be seen here, from where I copy-pasted it as follows:

unsigned divlu2(unsigned u1, unsigned u0, unsigned v,
                unsigned *r) {
   const unsigned b = 65536; // Number base (16 bits).
   unsigned un1, un0,        // Norm. dividend LSD's.
            vn1, vn0,        // Norm. divisor digits.
            q1, q0,          // Quotient digits.
            un32, un21, un10,// Dividend digit pairs.
            rhat;            // A remainder.
   int s;                    // Shift amount for norm.

   if (u1 >= v) {            // If overflow, set rem.
      if (r != NULL)         // to an impossible value,
         *r = 0xFFFFFFFF;    // and return the largest
      return 0xFFFFFFFF;}    // possible quotient.

   s = nlz(v);               // 0 <= s <= 31.
   v = v << s;               // Normalize divisor.
   vn1 = v >> 16;            // Break divisor up into
   vn0 = v & 0xFFFF;         // two 16-bit digits.

   un32 = (u1 << s) | (u0 >> 32 - s) & (-s >> 31);
   un10 = u0 << s;           // Shift dividend left.

   un1 = un10 >> 16;         // Break right half of
   un0 = un10 & 0xFFFF;      // dividend into two digits.

   q1 = un32/vn1;            // Compute the first
   rhat = un32 - q1*vn1;     // quotient digit, q1.
again1:
   if (q1 >= b || q1*vn0 > b*rhat + un1) {
     q1 = q1 - 1;
     rhat = rhat + vn1;
     if (rhat < b) goto again1;}

   un21 = un32*b + un1 - q1*v;  // Multiply and subtract.

   q0 = un21/vn1;            // Compute the second
   rhat = un21 - q0*vn1;     // quotient digit, q0.
again2:
   if (q0 >= b || q0*vn0 > b*rhat + un0) {
     q0 = q0 - 1;
     rhat = rhat + vn1;
     if (rhat < b) goto again2;}

   if (r != NULL)            // If remainder is wanted,
      *r = (un21*b + un0 - q0*v) >> s;     // return it.
   return q1*b + q0;
}

Specifically, I'm interested in the bounds of the variable un21. How large can it be? Somewhat surprising, it can be larger than v but by how much?

In other words, under again2 there is the test q0 >= b. If I wanted to know whether the division (q0 = un21/vn1) eventually overflows, is it enough to test (un21 >> 16) == vn1 or does it have to read (un21 >> 16) >= vn1, instead if q0 >= b?

The idea is to know in advance, prior to calculating the quotient, whether the division overflows or not.