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

From: Michal Hocko
Date: Wed Mar 09 2016 - 07:56:59 EST

On Wed 09-03-16 13:18:50, Ingo Molnar wrote:
> * Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> > Hi,
> >
> > the following patchset implements a killable variant of write lock for
> > rw_semaphore. My usecase is to turn as many mmap_sem write users to use a
> > killable variant which will be helpful for the oom_reaper [1] to asynchronously
> > tear down the oom victim address space which requires mmap_sem for read. This
> > will reduce a likelihood of OOM livelocks caused by oom victim being stuck on a
> > lock or other resource which prevents it to reach its exit path and release the
> > memory. [...]
> So I'm a tiny bit concerned about this arguments.
> AFAICS killability here just makes existing system calls more interruptible -
> right?

see below

> In that sense that's not really a livelock scenario: it just takes shorter
> time for resources to be released.
> If a livelock is possible (where resources are never released) then I'd like to
> see a specific example of such a livelock.
> You have the other patch-set:
> [PATCH 0/18] change mmap_sem taken for write killable
> that makes use of down_write_killable(), and there you argue:
> [...] 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.

> So there's no livelock scenario - it's "only" about latencies.

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.

down_write_interruptible will be trivial to do on top.

Michal Hocko