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

From	Miroslav Benes <mbenes@suse.cz>
Newsgroups	linux.kernel
Subject	Re: [PATCH 3/3] livepatch: add shadow variable sample program
Date	2017-06-19 19:00 +0200
Message-ID	<tU8bU-5do-27@gated-at.bofh.it> (permalink)
References	(1 earlier) <tND18-68x-17@gated-at.bofh.it> <tSht0-6j2-9@gated-at.bofh.it> <tShW2-6u5-15@gated-at.bofh.it> <tSAP0-1J4-7@gated-at.bofh.it> <tSDjP-36J-1@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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> > > 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

Thread

Re: [PATCH 3/3] livepatch: add shadow variable sample program Petr Mladek <pmladek@suse.com> - 2017-06-14 16:30 +0200
  Re: [PATCH 3/3] livepatch: add shadow variable sample program Josh Poimboeuf <jpoimboe@redhat.com> - 2017-06-14 17:00 +0200
    Re: [PATCH 3/3] livepatch: add shadow variable sample program Petr Mladek <pmladek@suse.com> - 2017-06-15 13:10 +0200
      Re: [PATCH 3/3] livepatch: add shadow variable sample program Josh Poimboeuf <jpoimboe@redhat.com> - 2017-06-15 15:50 +0200
        Re: [PATCH 3/3] livepatch: add shadow variable sample program Josh Poimboeuf <jpoimboe@redhat.com> - 2017-06-15 17:30 +0200
        Re: [PATCH 3/3] livepatch: add shadow variable sample program Miroslav Benes <mbenes@suse.cz> - 2017-06-19 19:00 +0200
  Re: [PATCH 3/3] livepatch: add shadow variable sample program Joe Lawrence <joe.lawrence@redhat.com> - 2017-06-14 17:20 +0200
    Re: [PATCH 3/3] livepatch: add shadow variable sample program Miroslav Benes <mbenes@suse.cz> - 2017-06-19 18:50 +0200

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