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

[Andrew Morton]

On Fri, 13 Nov 2020 16:06:25 -0800 Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:

> On Fri, Nov 13, 2020 at 3:55 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Fri, 13 Nov 2020 09:34:48 -0800 Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> >
> > > When a process is being killed it might be in an uninterruptible sleep
> > > which leads to an unpredictable delay in its memory reclaim. In low memory
> > > situations, when it's important to free up memory quickly, such delay is
> > > problematic. Kernel solves this problem with oom-reaper thread which
> > > performs memory reclaim even when the victim process is not runnable.
> > > Userspace currently lacks such mechanisms and the need and potential
> > > solutions were discussed before (see links below).
> > > This patch provides a mechanism to perform memory reclaim in the context
> > > of the process that sends SIGKILL signal. New SYNC_REAP_MM flag for
> > > pidfd_send_signal syscall can be used only when sending SIGKILL signal
> > > and will lead to the caller synchronously reclaiming the memory that
> > > belongs to the victim and can be easily reclaimed.
> >
> > hm.
> >
> > 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@xxxxxxxxxxxxxx
> . 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)?

There may be things which trip this up, such as mlocked regions or
whatever, but we could add another madvise `advice' mode to handle
this?