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

On Fri, 13 Nov 2020 16:06:25 -0800 Suren Baghdasaryan [off-list ref] wrote:

On Fri, Nov 13, 2020 at 3:55 PM Andrew Morton [off-list ref] wrote:

quoted

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.

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 (local)
. 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?