Consider the system \[x+y=z+u,\] \[2xy=zu.\] Find the greatest value of the real constant $m$ such that $m \le \frac{x}{y}$ for any positive integer solution $(x, y, z, u)$ of the system, with $x \ge y$.
1. **Given System of Equations:**
   \[
   x + y = z + u
   \]
   \[
   2xy = zu
   \]
   We need to find the greatest value of the real constant \( m \) such that \( m \le \frac{x}{y} \) for any positive integer solution \((x, y, z, u)\) of the system, with \( x \ge y \).

2. **Analyzing the Solutions:**
   We are given that all solutions \((x, y, z, u)\) such that \(\gcd(z, u) = d\), \(\frac{z}{d}\) is odd, and \(x > y\) are in the form:
   \[
   (x, y, z, u) = d(m(m+n), n(m-n), m^2 - n^2, 2mn)
   \]
   where \(m\) and \(n\) are coprime positive integers with different parities such that \(m > n\).

3. **Expressing the Ratio \(\frac{x}{y}\):**
   Let \(q = \frac{m}{n}\). Then:
   \[
   x = d \cdot m(m+n) = d \cdot n \cdot q(q+1)
   \]
   \[
   y = d \cdot n(m-n) = d \cdot n(q-1)
   \]
   Therefore:
   \[
   \frac{x}{y} = \frac{d \cdot n \cdot q(q+1)}{d \cdot n(q-1)} = \frac{q(q+1)}{q-1}
   \]

4. **Optimizing the Ratio:**
   We need to find the minimum value of \(\frac{q(q+1)}{q-1}\) for \(q > 1\). Let:
   \[
   f(q) = \frac{q(q+1)}{q-1}
   \]
   Simplifying \(f(q)\):
   \[
   f(q) = \frac{q^2 + q}{q-1} = q + 1 + \frac{q + 1}{q-1}
   \]
   To find the minimum value, we can use calculus. First, find the derivative \(f'(q)\):
   \[
   f'(q) = \frac{d}{dq} \left( \frac{q^2 + q}{q-1} \right)
   \]
   Using the quotient rule:
   \[
   f'(q) = \frac{(2q + 1)(q-1) - (q^2 + q)(1)}{(q-1)^2}
   \]
   Simplifying the numerator:
   \[
   (2q + 1)(q-1) - (q^2 + q) = 2q^2 - 2q + q - 1 - q^2 - q = q^2 - 2q - 1
   \]
   Thus:
   \[
   f'(q) = \frac{q^2 - 2q - 1}{(q-1)^2}
   \]
   Setting \(f'(q) = 0\):
   \[
   q^2 - 2q - 1 = 0
   \]
   Solving this quadratic equation:
   \[
   q = \frac{2 \pm \sqrt{4 + 4}}{2} = 1 \pm \sqrt{2}
   \]
   Since \(q > 1\), we take:
   \[
   q = 1 + \sqrt{2}
   \]

5. **Calculating the Minimum Value:**
   Substituting \(q = 1 + \sqrt{2}\) into \(f(q)\):
   \[
   f(1 + \sqrt{2}) = \frac{(1 + \sqrt{2})(2 + \sqrt{2})}{\sqrt{2}} = \frac{(1 + \sqrt{2})(2 + \sqrt{2})}{\sqrt{2}}
   \]
   Simplifying:
   \[
   f(1 + \sqrt{2}) = \frac{2 + 3\sqrt{2} + 2}{\sqrt{2}} = \frac{4 + 3\sqrt{2}}{\sqrt{2}} = 2\sqrt{2} + 3
   \]

Thus, the greatest value of \(m\) such that \(m \le \frac{x}{y}\) for any positive integer solution \((x, y, z, u)\) of the system, with \(x \ge y\), is \(3 + 2\sqrt{2}\).

The final answer is \(\boxed{3 + 2\sqrt{2}}\)