linux-kernel - Re: [PATCH 1/1] RFC: add pidfd_send_signal flag to reclaim mm while killing a process

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Message-ID: <20201118193233.GV12284@dhcp22.suse.cz>
Date:   Wed, 18 Nov 2020 20:32:33 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <guro@...com>, Rik van Riel <riel@...riel.com>,
        Christian Brauner <christian@...uner.io>,
        Oleg Nesterov <oleg@...hat.com>,
        Tim Murray <timmurray@...gle.com>, linux-api@...r.kernel.org,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...roid.com>,
        Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH 1/1] RFC: add pidfd_send_signal flag to reclaim mm while
 killing a process

On Wed 18-11-20 11:22:21, Suren Baghdasaryan wrote:
> On Wed, Nov 18, 2020 at 11:10 AM Michal Hocko <mhocko@...e.com> wrote:
> >
> > On Fri 13-11-20 18:16:32, Andrew Morton wrote:
> > [...]
> > > It's all sounding a bit painful (but not *too* painful).  But to
> > > reiterate, I do think that adding the ability for a process to shoot
> > > down a large amount of another process's memory is a lot more generally
> > > useful than tying it to SIGKILL, agree?
> >
> > I am not sure TBH. Is there any reasonable usecase where uncoordinated
> > memory tear down is OK and a target process which is able to see the
> > unmapped memory?
> 
> I think uncoordinated memory tear down is a special case which makes
> sense only when the target process is being killed (and we can enforce
> that by allowing MADV_DONTNEED to be used only if the target process
> has pending SIGKILL).

That would be safe but then I am wondering whether it makes sense to
implement as a madvise call. It is quite strange to expect somebody call
a syscall on a killed process. But this is more a detail. I am not a
great fan of a more generic MADV_DONTNEED on a remote process. This is
just too dangerous IMHO.

> However, the ability to apply other flavors of
> process_madvise() to large memory areas spanning multiple VMAs can be
> useful in more cases.

Yes I do agree with that. The error reporting would be more tricky but
I am not really sure that the exact reporting is really necessary for
advice like interface.

> For example in Android we will use
> process_madvise(MADV_PAGEOUT) to "shrink" an inactive background
> process.

That makes sense to me.
-- 
Michal Hocko
SUSE Labs