Re: [PATCH v3 1/2] mm: introduce process_mrelease system call

From: Suren Baghdasaryan <surenb@google.com>
To: Shakeel Butt <shakeelb@google.com>
Cc: Michal Hocko <mhocko@suse.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	David Rientjes <rientjes@google.com>,
	Matthew Wilcox <willy@infradead.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Roman Gushchin <guro@fb.com>, Rik van Riel <riel@surriel.com>,
	Minchan Kim <minchan@kernel.org>,
	Christian Brauner <christian@brauner.io>,
	Christoph Hellwig <hch@infradead.org>,
	Oleg Nesterov <oleg@redhat.com>,
	David Hildenbrand <david@redhat.com>,
	Jann Horn <jannh@google.com>, Andy Lutomirski <luto@kernel.org>,
	Christian Brauner <christian.brauner@ubuntu.com>,
	Florian Weimer <fweimer@redhat.com>,
	Jan Engelhardt <jengelh@inai.de>,
	Tim Murray <timmurray@google.com>,
	Linux API <linux-api@vger.kernel.org>,
	Linux MM <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>,
	kernel-team <kernel-team@android.com>
Subject: Re: [PATCH v3 1/2] mm: introduce process_mrelease system call
Date: Fri, 23 Jul 2021 09:08:50 -0700	[thread overview]
Message-ID: <CAJuCfpGmpwTv92joNuVPaEJg1PigtGQn2daywHaqF4TXjuiCWQ@mail.gmail.com> (raw)
In-Reply-To: <CALvZod4=9aEd9tUdku293uhVQ4mqsfYckCOKzqxXVTDYsmaVtQ@mail.gmail.com>

On Fri, Jul 23, 2021 at 6:46 AM Shakeel Butt <shakeelb@google.com> wrote:
>
> On Fri, Jul 23, 2021 at 1:53 AM Michal Hocko <mhocko@suse.com> wrote:
> >
> [...]
> > > However
> > > retrying means issuing another syscall, so additional overhead...
> > > I guess such "best effort" approach would be unusual for a syscall, so
> > > maybe we can keep it as it is now and if such "do not block" mode is needed
> > > we can use flags to implement it later?
> >
> > Yeah, an explicit opt-in via flags would be an option if that turns out
> > to be really necessary.
> >
>
> I am fine with keeping it as it is but we do need the non-blocking
> option (via flags) to enable userspace to act more aggressively.

I think you want to check memory conditions shortly after issuing
kill/reap requests irrespective of mmap_sem contention. The reason is
that even when memory release is not blocked, allocations from other
processes might consume memory faster than we release it. For example,
in Android we issue kill and start waiting on pidfd for its death
notification. As soon as the process is dead we reassess the situation
and possibly kill again. If the process is not dead within a
configurable timeout we check conditions again and might issue more
kill requests (IOW our wait for the process to die has a timeout). If
process_mrelease() is blocked on mmap_sem, we might timeout like this.
I imagine that a non-blocking option for process_mrelease() would not
really change this logic.
Adding such an option is trivial but I would like to make sure it's
indeed useful. Maybe after the syscall is in place you can experiment
with it and see if such an option would really change the way you use
it?