• Luis Chamberlain's avatar
    module: add debugging auto-load duplicate module support · 8660484e
    Luis Chamberlain authored
    The finit_module() system call can in the worst case use up to more than
    twice of a module's size in virtual memory. Duplicate finit_module()
    system calls are non fatal, however they unnecessarily strain virtual
    memory during bootup and in the worst case can cause a system to fail
    to boot. This is only known to currently be an issue on systems with
    larger number of CPUs.
    
    To help debug this situation we need to consider the different sources for
    finit_module(). Requests from the kernel that rely on module auto-loading,
    ie, the kernel's *request_module() API, are one source of calls. Although
    modprobe checks to see if a module is already loaded prior to calling
    finit_module() there is a small race possible allowing userspace to
    trigger multiple modprobe calls racing against modprobe and this not
    seeing the module yet loaded.
    
    This adds debugging support to the kernel module auto-loader (*request_module()
    calls) to easily detect duplicate module requests. To aid with possible bootup
    failure issues incurred by this, it will converge duplicates requests to a
    single request. This avoids any possible strain on virtual memory during
    bootup which could be incurred by duplicate module autoloading requests.
    
    Folks debugging virtual memory abuse on bootup can and should enable
    this to see what pr_warn()s come on, to see if module auto-loading is to
    blame for their wores. If they see duplicates they can further debug this
    by enabling the module.enable_dups_trace kernel parameter or by enabling
    CONFIG_MODULE_DEBUG_AUTOLOAD_DUPS_TRACE.
    
    Current evidence seems to point to only a few duplicates for module
    auto-loading. And so the source for other duplicates creating heavy
    virtual memory pressure due to larger number of CPUs should becoming
    from another place (likely udev).
    Signed-off-by: default avatarLuis Chamberlain <mcgrof@kernel.org>
    8660484e
Kconfig 14.5 KB