Re: [PATCH v8] oom_kill.c: futex: Don't OOM reap the VMA containing the robust_list_head

From: Thomas Gleixner <tglx@linutronix.de>
To: Michal Hocko <mhocko@suse.com>
Cc: Joel Savitz <jsavitz@redhat.com>, Nico Pache <npache@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	linux-mm@kvack.org, linux-kernel <linux-kernel@vger.kernel.org>,
	Rafael Aquini <aquini@redhat.com>,
	Waiman Long <longman@redhat.com>, Baoquan He <bhe@redhat.com>,
	Christoph von Recklinghausen <crecklin@redhat.com>,
	Don Dutile <ddutile@redhat.com>,
	"Herton R . Krzesinski" <herton@redhat.com>,
	David Rientjes <rientjes@google.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Davidlohr Bueso <dave@stgolabs.net>,
	Ingo Molnar <mingo@redhat.com>,
	Darren Hart <dvhart@infradead.org>,
	stable@kernel.org
Subject: Re: [PATCH v8] oom_kill.c: futex: Don't OOM reap the VMA containing the robust_list_head
Date: Mon, 11 Apr 2022 09:47:14 +0200	[thread overview]
Message-ID: <87wnfwf4e5.ffs@tglx> (raw)
In-Reply-To: <YlPPRrGrX2ECe8QY@dhcp22.suse.cz>

Michal,

On Mon, Apr 11 2022 at 08:48, Michal Hocko wrote:
> On Fri 08-04-22 23:41:11, Thomas Gleixner wrote:
>> So why would a process private robust mutex be any different from a
>> process shared one?
>
> Purely from the OOM POV they are slightly different because the OOM
> killer always kills all threads which share the mm with the selected
> victim (with an exception of the global init - see __oom_kill_process).
> Note that this is including those threads which are not sharing signals
> handling.
> So clobbering private locks shouldn't be observable to an alive thread
> unless I am missing something.

Yes, it kills everything, but the reaper also reaps non-shared VMAs. So
if the process private futex sits in a reaped VMA the shared one becomes
unreachable.

> On the other hand I do agree that delayed oom_reaper execution is a
> reasonable workaround and the most simplistic one.

I think it's more than a workaround. It's a reasonable expectation that
the kernel side of the user space threads can mop up the mess the user
space part created. So even if one of of N threads is stuck in a place
where it can't, then N-1 can still reach do_exit() and mop their mess
up.

The oom reaper is the last resort to resolve the situation in case of a
stuck task. No?

> If I understand your example code then we would need to evaluate the
> whole robust list and that is simply not feasible because that would
> require a #PF in general case.

Right. The robust list exit code does the user access with pagefaults
disabled and if it fails, it terminates the list walk. Bad luck :)

Thanks,

        tglx