Re: [PATCH v15 08/11] livepatch: Remove Nop structures when unused

[Miroslav Benes]

On Wed, 9 Jan 2019, Petr Mladek wrote:

> Replaced patches are removed from the stack when the transition is
> finished. It means that Nop structures will never be needed again
> and can be removed. Why should we care?
> 
>   + Nop structures give the impression that the function is patched
>     even though the ftrace handler has no effect.
> 
>   + Ftrace handlers do not come for free. They cause slowdown that might
>     be visible in some workloads. The ftrace-related slowdown might
>     actually be the reason why the function is no longer patched in
>     the new cumulative patch. One would expect that cumulative patch
>     would help solve these problems as well.
> 
>   + Cumulative patches are supposed to replace any earlier version of
>     the patch. The amount of NOPs depends on which version was replaced.
>     This multiplies the amount of scenarios that might happen.
> 
>     One might say that NOPs are innocent. But there are even optimized
>     NOP instructions for different processors, for example, see
>     arch/x86/kernel/alternative.c. And klp_ftrace_handler() is much
>     more complicated.
> 
>   + It sounds natural to clean up a mess that is no longer needed.
>     It could only be worse if we do not do it.
> 
> This patch allows to unpatch and free the dynamic structures independently
> when the transition finishes.
> 
> The free part is a bit tricky because kobject free callbacks are called
> asynchronously. We could not wait for them easily. Fortunately, we do
> not have to. Any further access can be avoided by removing them from
> the dynamic lists.
> 
> Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>

Acked-by: Miroslav Benes <mbenes@xxxxxxx>

Miroslav