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

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Date:   Fri, 13 Nov 2020 17:18:10 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Michal Hocko <mhocko@...nel.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 Fri, 13 Nov 2020 17:09:37 -0800 Suren Baghdasaryan <surenb@...gle.com> wrote:

> > > > Seems to me that the ability to reap another process's memory is a
> > > > generally useful one, and that it should not be tied to delivering a
> > > > signal in this fashion.
> > > >
> > > > And we do have the new process_madvise(MADV_PAGEOUT).  It may need a
> > > > few changes and tweaks, but can't that be used to solve this problem?
> > >
> > > Thank you for the feedback, Andrew. process_madvise(MADV_DONTNEED) was
> > > one of the options recently discussed in
> > > https://lore.kernel.org/linux-api/CAJuCfpGz1kPM3G1gZH+09Z7aoWKg05QSAMMisJ7H5MdmRrRhNQ@mail.gmail.com
> > > . The thread describes some of the issues with that approach but if we
> > > limit it to processes with pending SIGKILL only then I think that
> > > would be doable.
> >
> > Why would it be necessary to read /proc/pid/maps?  I'd have thought
> > that a starting effort would be
> >
> >         madvise((void *)0, (void *)-1, MADV_PAGEOUT)
> >
> > (after translation into process_madvise() speak).  Which is equivalent
> > to the proposed process_madvise(MADV_DONTNEED_MM)?
> 
> Yep, this is very similar to option #3 in
> https://lore.kernel.org/linux-api/CAJuCfpGz1kPM3G1gZH+09Z7aoWKg05QSAMMisJ7H5MdmRrRhNQ@mail.gmail.com
> and I actually have a tested prototype for that.

Why is the `vector=NULL' needed?  Can't `vector' point at a single iovec
which spans the whole address range?

> If that's the
> preferred method then I can post it quite quickly.

I assume you've tested that prototype.  How did its usefulness compare
with this SIGKILL-based approach?