linux-kernel - Re: [RFC 0/12] introduce down_write_killable for rw

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Message-ID: <20160309131710.GB7978@gmail.com>
Date:	Wed, 9 Mar 2016 14:17:10 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	"David S. Miller" <davem@...emloft.net>,
	Tony Luck <tony.luck@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Zankel <chris@...kel.net>,
	Max Filippov <jcmvbkbc@...il.com>, x86@...nel.org,
	linux-alpha@...r.kernel.org, linux-ia64@...r.kernel.org,
	linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
	sparclinux@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
	linux-arch@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [RFC 0/12] introduce down_write_killable for rw_semaphore


* Michal Hocko <mhocko@...nel.org> 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)

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!

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

Thanks,

	Ingo