Messages in this thread | | | From | (Eric W. Biederman) | Date | Thu, 11 Nov 2021 13:20:11 -0600 | Subject | Re: [RFC v1 2/4] kernel/fork.c: implement new process_mmput_async syscall |
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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.
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.
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.
This whole thing seems an exercise in spending lots of resources to free resources much later.
Eric
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