Re: [RFC]: userspace memory reaping

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Oct 15, 2020 at 11:43 AM Minchan Kim <minchan@kernel.org> wrote:
>
> On Thu, Oct 15, 2020 at 11:20:30AM +0200, Michal Hocko wrote:
>
> > > > I do have a vague recollection that we have discussed a kill(2) based
> > > > approach as well in the past. Essentially SIG_KILL_SYNC which would
> > > > not only send the signal but it would start a teardown of resources
> > > > owned by the task - at least those we can remove safely. The interface
> > > > would be much more simple and less tricky to use. You just make your
> > > > userspace oom killer or potentially other users call SIG_KILL_SYNC which
> > > > will be more expensive but you would at least know that as many
> > > > resources have been freed as the kernel can afford at the moment.
> > >
> > > Correct, my early RFC here
> > > https://patchwork.kernel.org/project/linux-mm/patch/20190411014353.113252-3-surenb@google.com
> > > was using a new flag for pidfd_send_signal() to request mm reaping by
> > > oom-reaper kthread. IIUC you propose to have a new SIG_KILL_SYNC
> > > signal instead of a new pidfd_send_signal() flag and otherwise a very
> > > similar solution. Is my understanding correct?
> >
> > Well, I think you shouldn't focus too much on the oom-reaper aspect
> > of it. Sure it can be used for that but I believe that a new signal
> > should provide a sync behavior. People more familiar with the process
> > management would be better off defining what is possible for a new sync
> > signal.  Ideally not only pro-active process destruction but also sync
> > waiting until the target process is released so that you know that once
> > kill syscall returns the process is gone.
>
> If we approach with signal, I am not sure we need to create new signal
> rather than pidfd and fsync(2) semantic.
>
> Furthermore, process_madvise makes the work in the caller context but
> signal might work somewhere else context depending on implemenation(
> oom reaper or CPU resumed the task). I am not sure it it fulfils Suren's
> requirement.
>
> One more thing to think over: Even though we spent some overhead to
> read /proc/pid/maps, we could make zapping in parallel in userspace
> with multi thread approach. I am not sure what's the win since Suren
> also care about zapping performance.

Sorry Minchan, I did not see your reply while replying to Michal...
Even if we do the reading/reaping in parallel, we still have to issue
10s of read() syscalls to consume the entire /proc/pid/maps file. Plus
I'm not sure how much mmap_sem contention such parallel operation
(reaping taking write lock and maps reading taking read lock) would
generate. If we go this route I think a syscall to read a vector of
VMAs would be way more performant and userspace usage would be much
simpler.