While trying to estimate the amount of memory consumed by a kernel module (usually device drivers),I tried using the size utility which gave the size of the static memory areas of the .ko ( .bss, .data, .text etc). So I was expecting the sum of these values to be exactly equal to the output given by the lsmod command immediately after inserting the module.
No dynamic memory allocation(kmalloc or vmalloc) is performed in the init() function to ensure that it isn’t causing the difference.So why is there a mismatch?
Curiously the mismatch was found to be a fixed amount most of the time!!
The command outputs are listed below
size chardev.ko
text data bss dec hex filename 172 448 1024016 1024636 fa27c chardev.ko lsmod
Module Size Used by Tainted: P chardev 1025040 0 - Live 0xc009d000
You mention that no allocation is done in the init function, but does that take into account calls such as register_chrdev(9) which allocate memory internally for the device instance? The comment that it is a constant difference makes me wonder if this might be the cause.