linux-kernel - Re: [RFC PATCH 0/2] improve vmalloc allocation

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Date:   Tue, 23 Oct 2018 14:01:58 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Shuah Khan <shuah@...nel.org>, Michal Hocko <mhocko@...nel.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Garnier <thgarnie@...gle.com>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joelaf@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...e.hu>, Tejun Heo <tj@...nel.org>,
        maco@...roid.com
Subject: Re: [RFC PATCH 0/2] improve vmalloc allocation

On Tue, Oct 23, 2018 at 01:09:23PM -0700, Matthew Wilcox wrote:
> On Tue, Oct 23, 2018 at 01:48:32PM -0600, Shuah Khan wrote:
> > On 10/23/2018 01:30 PM, Joel Fernandes wrote:
> > > On Tue, Oct 23, 2018 at 11:13:36AM -0600, Shuah Khan wrote:
> > >> I like this proposal. I think we will open up lot of test opportunities with
> > >> this approach.
> > >>
> > >> Maybe we can use this stress test as a pilot and see where it takes us.
> > > 
> > > I am a bit worried that such an EXPORT_SYMBOL_KSELFTEST mechanism can be abused by
> > > out-of-tree module writers to call internal functionality.
> > 
> > That is  valid concern to consider before we go forward with the proposal.
> > 
> > We could wrap EXPORT_SYMBOL_KSELFTEST this in an existing debug option. This could
> > be fine grained for each sub-system for its debug option. We do have a few of these
> > now
> 
> This all seems far more complicated than my proposed solution.

Matthew's solution seems Ok to me where it works. A problem could be that it
will not always work.

As an example, recently I wanted to directly set the sysctl_sched_rt_runtime
variable from the rcutorture test, just for forcing some conditions. This
symbol is internal and inaccessible from modules. This can also be done by
calling the internal sched_rt_handler with some parameters.  However I don't
think including an internal source file in a test source file can achieve the
objective of setting it since access to the internal symbol is not possible
without exporting it somehow. This could be a "special" case too but is an
example where the include trick may fall apart.

I do think its a cool trick though ;-)

 - Joel