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SubjectRe: [PATCH v15 08/11] livepatch: Remove Nop structures when unused
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@suse.com>

Acked-by: Miroslav Benes <mbenes@suse.cz>

Miroslav

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