Understanding the Linux Virtual Memory Manager
3 Page Table Management
Describing Physical Memory
/*
* On NUMA machines, each NUMA node would have a pg_data_t to describe
* it's memory layout. On UMA machines there is a single pglist_data which
* describes the whole memory.
*
* Memory statistics and page replacement data structures are maintained on a
* per-zone basis.
*/
typedef struct pglist_data {
struct zone node_zones[MAX_NR_ZONES];
#ifdef CONFIG_FLAT_NODE_MEM_MAP /* means !SPARSEMEM */
struct page *node_mem_map;
#endif
#ifndef CONFIG_NO_BOOTMEM
struct bootmem_data *bdata;
#endif
} pg_data_t;
- 4.3 Process Address Space Descriptor
The process address space is described by the mm_struct struct meaning that only one exists for each process and is shared between userspace threads. In fact, threads are identified in the task list by finding all task_structs which have pointers to the same mm_struct.
- 4.4 Memory Regions
Each region is represented by a vm_area_struct which never overlap and represent a set of addresses with the same protection and purpose. Examples of a region include a read-only shared library loaded into the address space or the process heap. A full list of mapped regions a process has may be viewed via the proc interface at /proc/PID/maps where PID is the process ID of the process that is to be examined
e.g /proc/3981/maps
b7520000-b7523000 r-xp 00000000 08:01 527837 /lib/i386-linux-gnu/libdl-2.19.so
b7523000-b7524000 r--p 00002000 08:01 527837 /lib/i386-linux-gnu/libdl-2.19.so
b7524000-b7525000 rw-p 00003000 08:01 527837 /lib/i386-linux-gnu/libdl-2.19.so
If the region is backed by a file, the struct file is available through the vm_file field which has a pointer to the struct inode. The inode is used to get the struct address_space which has all the private information about the file including a set of pointers to filesystem functions which perform the filesystem specific operations such as reading and writing pages to disk.
./include/linux/mm_types.h
struct vm_area_struct {
struct mm_struct *vm_mm; /* The address space we belong to. */
struct file * vm_file; /* File we map to (can be NULL). */
};
./include/linux/fs.h
struct file {
struct inode *f_inode; /* cached value */
};
struct inode {
struct address_space *i_mapping;
};
