Re: [RFC 0/12] introduce down_write_killable for rw_semaphore

From: Michal Hocko
Date: Wed Mar 09 2016 - 08:29:01 EST

On Wed 09-03-16 14:17:10, Ingo Molnar wrote:
> * Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> > > [...] this is a follow up work for oom_reaper [1]. As the async OOM killing
> > > depends on oom_sem for read we would really appreciate if a holder for write
> > > stood in the way. This patchset is changing many of down_write calls to be
> > > killable to help those cases when the writer is blocked and waiting for
> > > readers to release the lock and so help __oom_reap_task to process the oom
> > > victim.
> > >
> > > there seems to be a misunderstanding: if a writer is blocked waiting for
> > > readers then no new readers are allowed - the writer will get its turn the
> > > moment all existing readers drop the lock.
> >
> > Readers might be blocked e.g. on the memory allocation which cannot proceed due
> > to OOM. Such a reader might be operating on a remote mm.
> Doing complex allocations with the mm locked looks fragile no matter what: we
> should add debugging code that warns if allocations are done with a remote mm
> locked. (it should be trivial)

No matter how fragile is that it is not something non-existent. Just
have a look at use_mm for example. We definitely do not want to warn
about those, right?

> In fact people were thining about turning the mm semaphore into a rwlock - with
> that no blocking call should be possible with the lock held.
> So I maintain:
> > > So there's no livelock scenario - it's "only" about latencies.
> With a qualification: s/only/mostly ;-)
> > Latency is certainly one aspect of it as well because the sooner the mmap_sem
> > gets released for other readers to sooner the oom_reaper can tear down the
> > victims address space and release the memory and free up some memory so that we
> > do not have to wait for the victim to exit.
> >
> > > And once we realize that it's about latencies (assuming I'm right!), not about
> > > correctness per se, I'm wondering whether it would be a good idea to introduce
> > > down_write_interruptible(), instead of down_write_killable().
> >
> > I am not against interruptible variant as well but I suspect that some paths are
> > not expected to return EINTR. I haven't checked them for this but killable is
> > sufficient for the problem I am trying to solve. That problem is real while
> > latencies do not seem to be that eminent.
> If they don't expect EINTR then they sure don't expect SIGKILL either!

Why? Each syscall already is killable as the task might be killed by the
OOM killer.

> There's a (very) low number of system calls that are not interruptible, but the
> vast majority is.

That might be true. I just fail to see how this is related to the
particular problem I am trying to solve. As I've said those callsites
which cause problems with latencies can be later converted to
interruptible waiting trivially.

Michal Hocko
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