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

From: Michal Hocko <mhocko@suse.com>
To: Suren Baghdasaryan <surenb@google.com>
Cc: Shakeel Butt <shakeelb@google.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 10:53:50 +0200	[thread overview]
Message-ID: <YPqDnqULylkkzQG5@dhcp22.suse.cz> (raw)
In-Reply-To: <CAJuCfpGSZwVgZ=FxhCV-uC_mzC7O-v-3k3tm-F6kOB7WM9t9tw@mail.gmail.com>

On Fri 23-07-21 01:11:51, Suren Baghdasaryan wrote:
> On Thu, Jul 22, 2021, 11:20 PM Michal Hocko <mhocko@suse.com> wrote:
> 
> > On Thu 22-07-21 21:47:56, Suren Baghdasaryan wrote:
> > > On Thu, Jul 22, 2021, 7:04 PM Shakeel Butt <shakeelb@google.com> wrote:
> > >
> > > > On Thu, Jul 22, 2021 at 6:14 PM Suren Baghdasaryan <surenb@google.com>
> > > > wrote:
> > > > >
> > > > [...]
> > > > > +
> > > > > +       mmap_read_lock(mm);
> > > >
> > > > How about mmap_read_trylock(mm) and return -EAGAIN on failure?
> > > >
> > >
> > > That sounds like a good idea. Thanks! I'll add that in the next respin.
> >
> > Why is that a good idea? Can you do anything meaningful about the
> > failure other than immediately retry the syscall and hope for the best?
> >
> 
> I was thinking if this syscall implements "best effort without blocking"
> approach then for a more strict usage user can simply retry.

I do not think we really want to promise non blocking behavior at this
stage unless that is absolutely necessary. The current implementation
goes an extra mile to not block but I wouldn't carve it into stone via
userspace expectations.

> 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.
-- 
Michal Hocko
SUSE Labs