Re: [PATCH 2/4 v0.5] sched/umcg: RFC: add userspace atomic helpers

On Tue, Sep 14, 2021 at 08:04:55PM +0200, Peter Zijlstra wrote:

On Tue, Sep 14, 2021 at 09:29:00AM -0700, Peter Oskolkov wrote:

quoted

In the version of the patchset that I'm preparing to send I've decided
to punt on the issue and just ask the userspace to deal with locking
the memory as it sees fit: mlock() is available and as far as I can

Sadly mlock() does not imply no faults. Someone had a too literal
reading of the POSIX-RT spec (of which mlock is part) and figured that
all that was required was to keep the page in memory, not avoid faults.

Linux has had this bahviour for ages, PREEMPT_RT has tried to change
this, but so far to no avail. At some point sys_mpin() was proposed to
meet the original POSIX-RT intent, but afaict that never actually
happened.

In short, mlock() does not avoid minor faults, or even migration faults,
which can take a fair while to resolve.

Also, even if it did, that would still not be acceptible because
userspace could fail to call mlock() at which point mis-behaving
userspace can deadlock the kernel, which is a no-no.