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

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Date:   Wed, 18 Nov 2020 11:55:31 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Michal Hocko <mhocko@...e.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, Nov 18, 2020 at 11:51 AM Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> On Wed, Nov 18, 2020 at 11:32 AM Michal Hocko <mhocko@...e.com> wrote:
> >
> > 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.
>
> Agree 100%

I assumed here that by "a more generic MADV_DONTNEED on a remote
process" you meant "process_madvise(MADV_DONTNEED) applied to a
process that is not being killed". Re-reading your comment I realized
that you might have meant "process_madvice() with generic support to
large memory areas". I hope I understood you correctly.

>
> >
> > > 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.
>
> Andrew's suggestion for this special mode to change return semantics
> to the usual "0 or error code" seems to me like the most reasonable
> way to deal with the return value limitation.
>
> >
> > > 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