io_uring_setup spuriously returning ENOMEM for one user

[Andres Freund <andres@anarazel.de>]

Hi,

While testing the error handling of my uring using postgres branch I
just encountered the situation that io_uring_setup() always fails with
ENOMEN.

It only does so for the user I did the testing on and not for other
users. During the testing a few io_uring using processes were kill -9'd
and a few core-dumped after abort(). No io_uring using processes are
still alive.

As the issue only happens to the one uid I suspect that
current_user()->locked_mem got corrupted, perhaps after hitting the
limit for real.

Unfortunately I do not see any way to debug this without restarting. It
seems the user wide limit isn't exported anywhere? I found
uids_sysfs_init() while grepping around, but it seems that's just a
leftover.

This happened on 07a56bb875afbe39dabbf6ba7b83783d166863db / 5.8rc5 +
16. I left the machine running for now, in case there's something that
can be debugged while running. But I've to restart it in a bit. It took
a while to hit this issue, unfortunately.

Greetings,

Andres Freund