Let x=y=z=2, we get m⩽23.
Below, we prove that the maximum value of m is 23.
Let x=a1−1,y=b1−1,z=c1−1, then we have
(a1−1)(b1−1)(c1−1)=a1+b1+c1−1.
Rearranging gives a+b+c=1.
Thus, the problem is transformed into proving:
1−aa+1−bb+1−cc+23⩽ab+bc+ca, i.e., 23⩽(ca−b+ca)+(ab−c+ab)+(bc−a+bc),
which is equivalent to 23⩽c(b+c)ab+a(c+a)bc+b(a+b)ca.
and c(b+c)ab+a(c+a)bc+b(a+b)ca=abc(b+c)(ab)2+abc(c+a)(bc)2+abc(a+b)(a)2⩾2abc(a+b+c)(ab+bc+ca)2=2abc(a+b+c)a2b2+b2c2+c2a2+2abc(a+b+c)⩾2abc(a+b+c)abc(a+b+c)+2abc(a+b+c)=23.
In conclusion, the maximum value of m is 23.