Checking Physically Contiguous Memory Allocation with kmalloc in the Linux Kernel

In the Linux kernel, kmalloc is a widely-used function for allocating memory. Typically, kmalloc allocates memory that is physically contiguous. However, in certain scenarios, you may need to ensure that the allocated memory is indeed physically contiguous. This article provides insights and a practical example to verify the physical contiguity of memory allocated by kmalloc through code.

Understanding Physically Contiguous Memory in kmalloc

Physically contiguous memory refers to memory regions that are allocated as a single, continuous block from the physical address space of the system. This is important in certain kernel operations where performance and reliability are critical, such as direct memory mapping or DMA (Direct Memory Access).

Verifying Physically Contiguous Memory

To verify whether the memory allocated by kmalloc is physically contiguous, you can use a combination of kernel functions to check the physical addresses and page structures.

Example Implementation

The following sample code demonstrates how to verify the physical contiguity of memory allocated by kmalloc in a kernel module:

.toFloat static memory pool (kmalloc). GetPhysical Address of the allocated memory using virt_to_phys. Check Physical Contiguity through the PageCompound flag.

Here is the sample code for a kernel module:

Example Kernel Module Source Code

define SIZE 4096     // Size of memory to allocate
static int __init my_module_init(void) {
    void *ptr;
    unsigned long phys_addr;
    struct page *page;
    // Allocate memory using kmalloc
    ptr  kmalloc(SIZE, GFP_KERNEL);
    if (!ptr) {
        printk(KERN_ERR "memory allocation failed
");
        return -ENOMEM;
    }
    // Get the physical address
    phys_addr  virt_to_phys(ptr);
    printk(KERN_INFO "Physical address: %lx
", phys_addr);
    // Check if the memory is physically contiguous
    page  virt_to_page(ptr);
    if (PageCompound(page)) {
        printk(KERN_INFO "Memory is physically contiguous
");
    } else {
        printk(KERN_INFO "Memory is not physically contiguous
");
    }
    // Free the allocated memory
    kfree(ptr);
    return 0;
}
static void __exit my_module_exit(void) {
    printk(KERN_INFO "Module unloaded
");
}
module_init(my_module_init);
module_exit(my_module_exit);
MODULE_LICENSE("GPL");
MODULE_AUTHOR("Your Name");
MODULE_DESCRIPTION("Example of checking kmalloc-allocated memory contiguity");

Explanation of the Code

1. kmalloc: This function is used to allocate a memory block of a specified size.

2. virt_to_phys: This function converts a virtual address to a physical address, which is essential for understanding the memory layout from the kernel’s perspective.

3. virt_to_page: This function returns the struct page structure that corresponds to the given virtual address, which is used in the following step.

4. PageCompound: This flag checks if the page is part of a compound page, which indicates that the memory may not be physically contiguous. In a simple case, a compound page typically represents a single memory allocation.

Note

1. This code must be compiled as a kernel module and loaded into the kernel. Ensure that you have the appropriate kernel headers and development environment set up for building kernel modules.

2. The check for physical contiguity is simplified. Depending on your specific requirements and kernel configuration, you may need to consider additional factors for a more accurate verification.

Conclusion

Ensuring that memory allocated by kmalloc is physically contiguous can be crucial for certain performance optimizations and reliability considerations in the Linux kernel. This article provides a basic method to verify this condition through kernel code. Experiment with the example provided and adapt it to your needs for more advanced scenarios.