Part.1

$ file /dev/mem
/dev/mem: character special (1/1)

➜  ~ man 4 mem
mem, kmem, port - system memory, kernel memory and system ports

/dev/mem is a character device file that is an image of the main memory 

of the computer.  

It may be used, for example, to examine (and even patch) the system.

Examining and patching is likely to lead to unexpected results 

when read-only or write-only bits are present.

Since  Linux 2.6.26, and depending on the architecture, 

the CONFIG_STRICT_DEVMEM kernel configuration option limits the areas 

which can be accessed through this file.  

For example: on x86, RAM access is not allowed but accessing 

memory-mapped PCI regions is.

// drivers/char/mem.c
881 static const struct memdev {
...
882     const char *name;
883     umode_t mode;
884     const struct file_operations *fops;
885     fmode_t fmode;
886 } devlist[] = {
887 #ifdef CONFIG_DEVMEM
888      [1] = { "mem", 0, &**mem_fops**, FMODE_UNSIGNED_OFFSET },
889 #endif
890 #ifdef CONFIG_DEVKMEM
891      [2] = { "kmem", 0, &kmem_fops, FMODE_UNSIGNED_OFFSET },
892 #endif
...
}

823 static const struct file_operations __maybe_unused mem_fops = {
**824     .llseek     = memory_lseek,
825     .read       = read_mem,
826     .write      = write_mem,**
827     .mmap       = mmap_mem,
828     .open       = open_mem,
829 #ifndef CONFIG_MMU
830     .get_unmapped_area = get_unmapped_area_mem,
831     .mmap_capabilities = memory_mmap_capabilities,
832 #endif
833 };

// drivers/char/mem.c
read_mem
	page_is_allowed
		devmem_is_allowed
	xlate_dev_mem_ptr
		memremap
	copy_to_user