Re: [RFC v1 2/4] kernel/fork.c: implement new process_mmput_async syscall

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On Thu, 11 Nov 2021 13:20:11 -0600
ebiederm@xmission.com (Eric W. Biederman) wrote:

> Claudio Imbrenda <imbrenda@linux.ibm.com> writes:
> 
> > The goal of this new syscall is to be able to asynchronously free the
> > mm of a dying process. This is especially useful for processes that use
> > huge amounts of memory (e.g. databases or KVM guests). The process is
> > allowed to terminate immediately, while its mm is cleaned/reclaimed
> > asynchronously.
> >
> > A separate process needs use the process_mmput_async syscall to attach
> > itself to the mm of a running target process. The process will then
> > sleep until the last user of the target mm has gone.
> >
> > When the last user of the mm has gone, instead of synchronously free
> > the mm, the attached process is awoken. The syscall will then continue
> > and clean up the target mm.
> >
> > This solution has the advantage that the cleanup of the target mm can
> > happen both be asynchronous and properly accounted for (e.g. cgroups).
> >
> > Tested on s390x.
> >
> > A separate patch will actually wire up the syscall.  
> 
> I am a bit confused.
> 
> You want the process report that it has finished immediately,
> and you want the cleanup work to continue on in the background.

yes

> Why do you need a separate process?
> 
> Why not just modify the process cleanup code to keep the task_struct
> running while allowing waitpid to reap the process (aka allowing
> release_task to run)?  All tasks can be already be reaped after
> exit_notify in do_exit.
> 
> I can see some reasons for wanting an opt-in.  It is nice to know all of
> a processes resources have been freed when waitpid succeeds.
> 
> Still I don't see why this whole thing isn't exit_mm returning
> the mm_sturct when a flag is set, and then having an exit_mm_late
> being called and passed the returned mm after exit_notify.

so if I understand correctly you are saying exit_mm would skip the
mmput, set a flag, then I should introduce a new function
"exit_mm_late" after exit_notify, to check the flag and do the mmput if
needed

and that would mean that the cleanup would still be done in the context
of the exiting process, but without holding back anyone waiting for the
process to terminate (so the process appears to exit immediately)

sounds clean, I will do it

> Or maybe something with schedule_work or task_work, instead of an
> exit_mm_late.  I don't see any practical difference.
> 
> I really don't see why this needs a whole other process to connect to
> the process you care about asynchronously.

accounting. workqueues or kernel threads are not properly accounted to
the right cgroups; by using a userspace process, things get accounted
properly.

this was a major point that was made last month when a similar
discussion came up

> This whole thing seems an exercise in spending lots of resources to free
> resources much later.

there are some usecases for this (huge processes like databases, or huge
secure VMs where the teardown is significantly slower than normal
processes)

> 
> Eric