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

From: Suren Baghdasaryan <surenb@google.com>
To: David Hildenbrand <david@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>,
	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>, 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 01:18:56 -0700	[thread overview]
Message-ID: <CAJuCfpFSi2eLQL1YkHOubX+53Aa1ngZ1hS2+QervFnt5dLEQSw@mail.gmail.com> (raw)
In-Reply-To: <23ed1d8d-fe55-fdbc-ca33-01a3ce392dff@redhat.com>

On Fri, Jul 23, 2021 at 1:15 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 23.07.21 10:11, Suren Baghdasaryan wrote:
> >
> >
> > On Thu, Jul 22, 2021, 11:20 PM Michal Hocko <mhocko@suse.com
> > <mailto: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
> >     <mailto:shakeelb@google.com>> wrote:
> >      >
> >      > > On Thu, Jul 22, 2021 at 6:14 PM Suren Baghdasaryan
> >     <surenb@google.com <mailto: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. 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?
>
> The process is dying, so I am not sure what we are trying to optimize
> here in respect to locking ...

Trying not to block the caller, which is likely a system health
monitoring process. However, if not blocking is important, it can
issue this syscall from a separate thread... Let's scratch that "do
not block" mode and keep it simple as it is now.

>
>
> --
> Thanks,
>
> David / dhildenb
>
> --
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