Re: [PATCH v3 1/2] mm: introduce process_mrelease system call

On Fri, Jul 23, 2021 at 9:09 AM Suren Baghdasaryan [off-list ref] wrote:

On Fri, Jul 23, 2021 at 6:46 AM Shakeel Butt [off-list ref] wrote:

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On Fri, Jul 23, 2021 at 1:53 AM Michal Hocko [off-list ref] wrote:

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However
retrying means issuing another syscall, so additional overhead...
I guess such "best effort" approach would be unusual for a syscall, so
maybe we can keep it as it is now and if such "do not block" mode is needed
we can use flags to implement it later?

Yeah, an explicit opt-in via flags would be an option if that turns out
to be really necessary.

I am fine with keeping it as it is but we do need the non-blocking
option (via flags) to enable userspace to act more aggressively.

I think you want to check memory conditions shortly after issuing
kill/reap requests irrespective of mmap_sem contention. The reason is
that even when memory release is not blocked, allocations from other
processes might consume memory faster than we release it. For example,
in Android we issue kill and start waiting on pidfd for its death
notification. As soon as the process is dead we reassess the situation
and possibly kill again. If the process is not dead within a
configurable timeout we check conditions again and might issue more
kill requests (IOW our wait for the process to die has a timeout). If
process_mrelease() is blocked on mmap_sem, we might timeout like this.
I imagine that a non-blocking option for process_mrelease() would not
really change this logic.

On a containerized system, killing a job requires killing multiple
processes and then process_mrelease() them. Now there is cgroup.kill
to kill all the processes in a cgroup tree but we would still need to
process_mrelease() all the processes in that tree. There is a chance
that we get stuck in reaping the early process. Making
process_mrelease() non-blocking will enable the userspace to go to
other processes in the list.

An alternative would be to have a cgroup specific interface for
reaping similar to cgroup.kill.

Adding such an option is trivial but I would like to make sure it's
indeed useful. Maybe after the syscall is in place you can experiment
with it and see if such an option would really change the way you use
it?

SGTM.