linux-kernel - Re: [PATCH] add hook to read_from_oldmem() to check for non-ram pages

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Date:	Fri, 6 May 2011 12:30:19 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Olaf Hering <olaf@...fle.de>
Cc:	linux-kernel@...r.kernel.org, kexec@...ts.infradead.org
Subject: Re: [PATCH] add hook to read_from_oldmem() to check for non-ram
 pages

On Fri, 6 May 2011 12:55:46 +0200
Olaf Hering <olaf@...fle.de> wrote:

> Should the called code increase/decrease the modules refcount instead?
> I remember there was some MODULE_INC/MODULE_DEC macro (cant remember the
> exact name) at some point. What needs to be done inside the module to
> prevent an unload while its still in use? Is it __module_get/module_put
> for each call of fn()? 

A try_module_get(THIS_MODULE) in the register function will do the trick.
However it's unneeded.  Documentation/DocBook/kernel-hacking.tmpl tells us

try_module_get() module_put()

    These manipulate the module usage count, to protect against
    removal (a module also can't be removed if another module uses one
    of its exported symbols: see below).  Before calling into module
    code, you should call <function>try_module_get()</function> on
    that module: if it fails, then the module is being removed and you
    should act as if it wasn't there.  Otherwise, you can safely enter
    the module, and call <function>module_put()</function> when you're
    finished.

So as your module will have a reference to vmcore.c's register and unregister
functions, nothing needs to be done: the presence of the client module alone
will pin the vmcore.c module.

However it's all moot, because the fs/proc/vmcore.c code cannot
presently be built as a module and it's rather unlikely that it ever
will be.

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