Re: OK to set PF_MEMDIE on cleanup tasks?

[Andrew Morton <akpm@osdl.org>]

"Paul E. McKenney" <paulmck@us.ibm.com> wrote:
>
> Hello!
> 
> We have tasks that actively return memory to the system, which we
> would like to exempt from the OOM killer, as killing such tasks under
> low-memory conditions would indeed be counterproductive.  It looks like
> the "official" way to do this is to catch/ignore signal 15, which results
> in PF_MEMDIE being set (in the 2.6 kernel), thus preventing the OOM killer
> from killing the task again.  I don't see where PF_MEMDIE is cleared,
> though there are a number of subtle ways one might do this that I would
> have missed.

The PF_MEMDIE flag is there so the oom killer doesn't just sit there
hitting the same task over and over again.

We leave PF_MEMDIE set because we expect the task to exit, or to not want
any more oomkiller attention.

The SIGTERM behaviour is there because the CAP_SYS_RAWIO process may need
to release critical resources.

So as long as your process has CAP_SYS_RAWIO, everything happens to work as
you want it.  I don't think it was really designed that way though.

> So...  Is it considered legit to simply set PF_MEMDIE when creating
> the cleanup task?  Or is there some reason that one should deal with
> signal 15?

Well it's all very unconventional.  Catching SIGTERM seems like a suitable
way to do what you want to do.

Possibly your special process should also run as PF_MEMALLOC.  I've seen
that done before, with success.  There is no existing API with which this
can be set.