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

[Miroslav Benes]

On Fri, 30 Jun 2017, Josh Poimboeuf wrote:

> On Mon, Jun 19, 2017 at 06:56:37PM +0200, Miroslav Benes wrote:
> > 
> > > > > 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.
> 
> We used to have something similar in kpatch.  And we recently discovered
> that this "replace_all" feature would also be nice to have in livepatch.
> 
> We had a patch B which needed to partially revert patch A.  We had to
> manually do the revert at a function level, which basically means
> repatching the function so that it resembles its original state.
> 
> It would be much more straightforward to be able to tell klp to revert
> everything in patch A while applying patch B.  Then the func stack would
> never have more than one entry.  And that would be good for performance
> as well.

Exactly.

It is on my TODO list right after the fake signal. I've been occupied by 
different things recently but I'll definitely return to it soon.

Regards,
Miroslav