Re: [PATCH v2] devres: Fix page faults when tracing devres from unloaded modules

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Fri, Sep 27, 2024 at 10:28:07PM +0800, Keita Morisaki wrote:
> The devres ftrace event logs the name of the devres node, which is often a
> function name (e.g., "devm_work_drop") stringified by macros like
> devm_add_action. Currently, ftrace stores this name as a string literal
> address, which can become invalid when the module containing the string is
> unloaded. This results in page faults when ftrace tries to access the name.
> 
> This behavior is problematic because the devres ftrace event is designed to
> trace resource management throughout a device driver's lifecycle, including
> during module unload. The event should be available even after the module
> is unloaded to properly diagnose resource issues.
> 
> Fix the issue by copying the devres node name into the ftrace ring buffer
> using __assign_str(), instead of storing just the address. This ensures
> that ftrace can always access the name, even if the module is unloaded.
> 
> This change increases the memory usage for each of the ftrace entry by
> 12-16 bytes assuming the average devres node name is 20 bytes long,
> depending on the size of const char *.
> 
> Note that this change does not affect anything unless all of following
> conditions are met.
> - CONFIG_DEBUG_DEVRES is enabled
> - ftrace tracing is enabled
> - The devres event is enabled in ftrace tracing

LGTM now,
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
thanks!

-- 
With Best Regards,
Andy Shevchenko