How to understand the macro __init?

[Ezequiel Garcia]

Hey Amar,

On Thu, Aug 16, 2012 at 3:39 PM, Amarnath Revanna
<amarnath.revanna at gmail.com> wrote:
> Just want to add a little more for better understanding:
>
> When I spoke about .init section of the final kernel image, please note that
> this section is going to
> contain all the __init data (and functions) coming from _All_ the drivers
> and modules that were included
> as part of the kernel image. Hence, after initialization when we look at the
> print:
>
> " [1.011596] Freeing unused kernel memory: 664k freed "
>
> we see 664k bytes being freed.
>
> This is a significant amount of contiguous physical memory that we can see
> being released by the kernel.
>
> The same cannot be held true for a single loadable module which may be
> releasing just a few, virtually
> contiguous memory.
>

It's crystal clear ;-) Nice explanation. It's important to add
something to clearify a bit your
explanation (please correct me if I'm wrong):

When Amar is talking about "virtually contiguous" kernel memory he
implies that this
memory is physically *dis*contiguous, i.e. based on page-entries.
This is the kind of memory used for loadable modules,
for instance, modules that get loaded with modprobe.

On the other hand, built-in modules are compiled *inside*  the kernel
image (bzImage).
The memory used for this image is physically contiguous: it's a big
contiguous block
of memory pages. Contiguous memory is important for kernel, and therefore
is *very* important to spend some effort minimizing it.

Regards,
Ezequiel.