linux-kernel

What are memory mapped page and anonymous page?

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The correct terms are memory mapped files and anonymous mappings. When referring to memory mapping, one is usually referring to mmap(2). There are 2 categories for using mmap. One category is SHARED vs PRIVATE mappings. The other category is FILE vs ANONYMOUS mappings. Mixed together you get the 4 following combinations: PRIVATE FILE MAPPING SHARED … Read more

kvm: module verification failed: signature and/or required key missing – tainting kernel

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Instead of re-configuring the kernel, this error (module verification failed) could be resolved by just adding one line CONFIG_MODULE_SIG=n to the top of the Makefile for the module itself: CONFIG_MODULE_SIG=n # If KERNELRELEASE is defined, we’ve been invoked from the # kernel build system and can use its language. ifneq ($(KERNELRELEASE),) obj-m := hello.o # … Read more

module_init() vs. core_initcall() vs. early_initcall()

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They determine the initialization order of built-in modules. Drivers will use device_initcall (or module_init; see below) most of the time. Early initialization (early_initcall) is normally used by architecture-specific code to initialize hardware subsystems (power management, DMAs, etc.) before any real driver gets initialized. Technical stuff for understanding below Look at init/main.c. After a few architecture-specific … Read more

Software memory bit-flip detection for platforms without ECC

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The thing is, ECC is dirt cheap compared to “software ECC countermeasures”. You can easily detect if they have ECC modules and complain (or print a warning) when they don’t. http://www.cyberciti.biz/faq/ecc-memory-modules/ For example, can we see all writes to memory (both from user and kernel space), to distinguish between intended memory changes from in-memory bit … Read more

What is the meaning of question marks ‘?’ in Linux kernel panic call traces?

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‘?’ means that the information about this stack entry is probably not reliable. The stack output mechanism (see the implementation of dump_trace() function) was unable to prove that the address it has found is a valid return address in the call stack. ‘?’ itself is output by printk_stack_address(). The stack entry may be valid or … Read more

What is the “current” in Linux kernel source?

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It’s a pointer to the current process (i.e. the process that issued the system call). On x86, it’s defined in arch/x86/include/asm/current.h (similar files for other archs). #ifndef _ASM_X86_CURRENT_H #define _ASM_X86_CURRENT_H #include <linux/compiler.h> #include <asm/percpu.h> #ifndef __ASSEMBLY__ struct task_struct; DECLARE_PER_CPU(struct task_struct *, current_task); static __always_inline struct task_struct *get_current(void) { return percpu_read_stable(current_task); } #define current get_current() #endif … Read more

Content for Linux Operating Systems Class

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My list: What an operating system’s concerns are: Abstraction and extension of the physical machine and resource management. How the build process works ie, how architecture specific/machine code stuff is implanted How system calls work and how modules can link up Memory management / Virtual Memory / Paging and all the rest How processes are … Read more