Developing a User Allocator-Deallocator Plugin for IMFS in RTEMS

In this blog, I share my experience developing a user-defined allocator-deallocator plugin for the IMFS filesystem in the RTEMS real-time operating system. This plugin enables custom memory management strategies, addressing limitations in the default RTEMS heap. Below, I outline the key components, implementation details, and lessons learned.

Why a User Allocator-Deallocator Plugin?

The RTEMS kernel typically relies on malloc and free for memory allocation and deallocation. However, the default heap can face issues like heap overflow, which can compromise system reliability in resource-constrained environments. The user allocator-deallocator plugin allows developers to define custom memory management strategies, reducing dependency on the RTEMS heap and mitigating risks like fragmentation or overflow.

The plugin integrates with the IMFS (In-Memory File System) by providing custom functions for allocation, deallocation, and querying free space. These are critical for operations like file creation and system calls such as statvfs.

Key Learnings

1. Configuring RTEMS with confdefs.h
   The confdefs.h header is a powerful tool for customizing RTEMS kernel behavior. By defining specific macros before including confdefs.h, you can override default kernel variables. For example, changing the block size used by IMFS.

2. Adding a Test Case to RTEMS Testsuites
   To validate the plugin, I created a new test case, psximfs03, in the RTEMS testsuite. This test case uses a freelist-based approach, where a large global array is divided into smaller blocks. These blocks are managed in a freelist, with the allocator removing blocks for use and the deallocator returning them to the freelist.

3. RTEMS Compilation and Linking
   Integrating the plugin required careful attention to RTEMS’s build system. The plugin code and test case were compiled and linked with the RTEMS kernel, ensuring compatibility with the IMFS and the custom memory management logic.

Implementing the IMFS_memfile_ops Structure

The core of the plugin is the IMFS_memfile_ops structure, which holds function pointers for three key operations:

• Allocate: This function is called whenever a data block is required while performing write, ftruncate, etc in file.
• Deallocate: This function is called whenever a data block is supposed to be remove.
• Get Free Space: Provides information for system calls like statvfs, reporting available memory.

Locating and changing the allocator functions used by kernel

• Modified functions memfile_alloc_block and memfile_free_block that calls the allocate and deallocate functions respectively.
• Used to calculate the f_bfree and f_bavail field in imfs_statvfs function

 

This structure allows IMFS to seamlessly call user-defined functions instead of relying on the default malloc and free. 

Test Case: psximfs03

The psximfs03 test case demonstrates the plugin’s functionality. I implemented a freelist by:

1. Declaring a large global array and dividing it into fixed-size blocks.
2. Inserting these blocks into a freelist during initialization.
3. Designing the allocator to remove a block from the freelist for allocation.
4. Designing the deallocator to return the block to the freelist.
5. Implementing a get_free_space function to track available blocks for system queries.

This approach ensures efficient memory management and allows the plugin to handle dynamic allocation requests while maintaining system stability.

 

Challenges and Insights

• Test Case Integration: Adding psximfs03 to the RTEMS testsuite involved understanding the suite’s structure and ensuring the test aligned with RTEMS conventions.
• Kernel Configuration: Properly defining macros in confdefs.h was critical to avoid conflicts with default settings.

Conclusion

Developing the user allocator-deallocator plugin for IMFS was a rewarding experience that deepened my understanding of RTEMS’s memory management and build system. By leveraging confdefs.h, creating a targeted test case, and implementing the IMFS_memfile_ops structure, I enabled custom memory management that enhances reliability in resource-constrained systems. This project highlights the flexibility of RTEMS and the power of user-defined plugins to address specific application needs.