In this book (understanding Linux kernel),

the kernel can easily obtain the address of the thread_info structure of the process currently running on a CPU from the value of the esp register. In fact, if the thread_union structure is 8 KB (213 bytes) long, the kernel masks out the 13 least significant bits of esp to obtain the base address of the thread_info structure; on the other hand, if the thread_union struc- ture is 4 KB long, the kernel masks out the 12 least significant bits of esp. This is done by the current_thread_info() function, which produces assembly language instructions like the following:

 movl $0xffffe000,%ecx or 0xfffff000 for 4KB stacks 
 andl %esp,%ecx
 movl %ecx,p

Why is "stack pointer(esp) & 0xffffe000" equal to the process descriptor base address?

That means the base address of process descriptor is always 0xXYZ...000, right? It is weird.