Re: [PATCH 3/3] livepatch: add shadow variable sample program

[Miroslav Benes <mbenes@suse.cz>]

> > > I often wonder whether it's really a good idea to even allow the
> > > unloading of patch modules at all.  It adds complexity to the livepatch
> > > code.  Is it worth it?  I don't have an answer but I'd be interested in
> > > other people's opinion.
> > 
> > I could imagine a situation when a livepatch causes, for example,
> > performance, problems on a server because of the redirection
> > to the new code. Then it might be handy to disable the patch
> > and ftrace handlers completely.
> 
> Fair enough, though it sounds theoretical.  It would be good to know
> we're supporting actual real world use cases.

We distribute cumulative "replace_all" patches at SUSE. replace_all means 
that all previous patches are reverted in the process of application. All 
livepatch modules with zero refcount are removed. This keeps a number of 
loaded modules low and system's state well defined, which is always a good 
thing, because a customer might run into problems and we'd have to debug 
it.

It is true that it is a limitation too. Especially for state changes and 
data structure modifications. Sometimes it is easy to patch a system, but 
impossible to unpatch it. Because we don't have a consistency on a state 
level, only on a task/process level. But I perceive this also as an 
advantage. I have to always know what a livepatch does exactly and I 
discovered couple of problems just because I had to think about unloading 
of modules.

Miroslav