And even if other threads are contending for the page table lock, the kernel doesn’t hold that lock for the entire duration of the I/O. Only for the tiny fraction of that duration where the kernel is spending CPU time doing bookkeeping. For the rest of the time, during which the system is just waiting for the disk, the thread that triggered the page-in is still blocked, but other threads can do whatever they want, including page-table modifications.

From what I’ve read on LWN, contention on the page table lock (mmap_sem / mmap_lock) has been a real and perennial issue for Linux, especially on servers with huge numbers of CPUs; but it’s a far smaller effect than what this post is talking about.