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

On Fri, Nov 13, 2020 at 03:55:39PM -0800, Andrew Morton wrote:

On Fri, 13 Nov 2020 09:34:48 -0800 Suren Baghdasaryan [off-list ref] wrote:

quoted

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.

I agree and I see you've already had some good ideas how to tie this to
process_madvise(). If that's workable for your use-case then I'd prefer
that approach. Signals are almost always not a great choice.

Christian