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

[Suren Baghdasaryan]

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.