Re: [RFC] ipc: introduce shm_rmid_forced sysctl

[Ingo Molnar <mingo@elte.hu>]

* Alan Cox <alan@lxorguk.ukuu.org.uk> wrote:

> > As we really prefer working systems over non-working ones (and lots 
> > of unattached shm segments can clearly result in a non-working 
> > system) we can only accept the "this will break stuff" argument if 
> > it's *demonstrated* to break stuff and if the failure scenario is 
> > carefully described in the commit.
> > 
> > It would take a serious breakage to override a "system locks up 
> > swapping itself to death" failure scenario.
> 
> Ths shared memory interface is defined to be persistent for good 
> reason and all sorts of apps rely upon that so no you can't just 
> ignore that. As a configurable alternative it makes sense (indeed 
> many SYS5 admins used to run shared memory segment sweepers to 
> clean up long idle ones)
> 
> However if it's locking the machine up and not being properly 
> handled by resource management then
> 
> a) your resource management is broken so fix that instead
> b) if your resource management is busted or you are not properly
> tracking resource commits then the user is going to be able to achieve the
> same result by other means (eg a unix domain socket bomb)
> 
> If you've got no overcommit set you shouldn't be able to swap to 
> death, it may be the sysv shared memory objects need to be 
> accounted for specifically somewhere but that would be the right 
> thing to fix and the mechanisms to do it exist.

But the majority of systems have overcommit enabled - that is our 
default.

This is a simple extension of the OOM killer being able to ... kill 
things on OOM, ok? 'to kill' implies 'to break'.

Thanks,

	Ingo