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    SubjectRe: [RFC PATCH 1/1] vmalloc: add test driver to analyse vmalloc allocator
    On Wed, 14 Nov 2018 16:17:37 +0100 Michal Hocko <mhocko@kernel.org> wrote:

    > On Tue 13-11-18 14:10:46, Andrew Morton wrote:
    > [...]
    > > > +static int vmalloc_test_init(void)
    > > > +{
    > > > + __my_vmalloc_node_range =
    > > > + (void *) kallsyms_lookup_name("__vmalloc_node_range");
    > > > +
    > > > + if (__my_vmalloc_node_range)
    > > > + do_concurrent_test();
    > > > +
    > > > + return -EAGAIN; /* Fail will directly unload the module */
    > > > +}
    > >
    > > It's unclear why this module needs access to the internal
    > > __vmalloc_node_range(). Please fully explain this in the changelog.
    > >
    > > Then, let's just export the thing. (I expect this module needs a
    > > Kconfig dependency on CONFIG_KALLSYMS, btw). A suitable way of doing
    > > that would be
    > >
    > > /* Exported for lib/test_vmalloc.c. Please do not use elsewhere */
    > > EXPORT_SYMBOL_GPL(__vmalloc_node_range);
    >
    > There was a previous discussion that testing for internal infrastructure
    > is useful quite often and such a testing module needs an access to such
    > an internal infrastructure. Exporting those symbols via standard
    > EXPORT_SYMBOL_GPL is far from optimal because we can be pretty much sure
    > an abuse will arise sooner than later. I was proposing
    > EXPORT_SYMBOL_SELFTEST that would link only against testing modules.

    That's rather overdoing things, I think. If someone uses a
    dont-use-this symbol then they get to own both pieces when it breaks.

    We could simply do

    #define EXPORT_SYMBOL_SELFTEST EXPORT_SYMBOL_GPL

    then write a script which checks the tree for usages of the
    thus-tagged symbols outside tools/testing and lib/ (?)

    > If that is not viable for some reason then kallsyms_lookup_name is a
    > dirty-but-usable workaround.

    Well yes. It adds a dependency on CONFIG_KALLSYMS and will cause
    silent breakage if __vmalloc_node_range gets renamed, has its arguments
    changed, etc.

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