why can't I edit /proc/sys/kernel/perf_event_max_sample_rate with vi

Question

I'm root, running Centos8 on bare mental and permissions on /proc/sys/kernel/perf_event_max_sample_rate are as follows

-rw-r--r--. 1 root root 0 Oct  3 14:00 perf_event_max_sample_rate

I want to change the sample rate by opening this file with vi, making the change and then writing it with either w or w!. When I do so it comes back with

perf_event_max_sample_rate" E667: Fsync failed

When I try to quit, it tells me

E37: No write since last change (add ! to override)

So I can't change the value with vi. However I can change the value with

echo 4000 > /proc/sys/kernel/perf_event_max_sample_rate

Is this simply because /proc is not a real file system and that all changes to things in /proc would have to be made using echo with a redirect, or maybe sed? Is this behavior documented somewhere?

Answer 1

This is because of the fsync option of vi/vim:

fsync fs
[…]
When on, the library function fsync() will be called after writing a file. This will flush a file to disk, ensuring that it is safely written even on filesystems which do metadata-only journaling. […]

^(source)

Apparently procfs doesn't support this. It doesn't need to. As it's an interface to internal data structures in the kernel, there is neither cache nor journaling. Permanent config is stored in /etc/sysctl.conf and few other locations related to sysctl.

My tests indicate the issue is not as straightforward as "unsupported call after writing a file". The failure prevents the change. Anyway, to make vi stop trying to fsync() tell it to set nofsync, then write with w. This works for me on Ubuntu 18.04.2 LTS.