Section 9.6. Anonymous Memory

9.6. Anonymous Memory

At many points we have mentioned anonymous memory. Anonymous memory refers to pages that are not directly associated with a vnode. Such pages are used for a process's heap space, its stack, and copy-on-write pages. In the Solaris kernel, two subsystems are dedicated to managing anonymous memory: the anon layer and the swapfs file system.

The anonymous memory allocated to the heap of a process is a result of a zero-fill-on-demand operation (ZFOD). The ZFOD operation is how we allocate new pages. A ZFOD occurs when we touch a memory address for the first time, and a new page of memory is dynamically allocated at that address. ZFOD is the allocation method used for the heap space and segments created as a map of /dev/zero with segment protection of MAP_PRIVATE. A page fault on a segment of this type will be recognized as ZFOD, and a new zeroed anonymous memory page is created at the fault location. The seg_vn segment fault handler, segvn_fault, handles the fault and creates ZFOD pages.

The seg_vn segment driver allocates anonymous memory to the segment by calling into the anonymous memory layer interfaces and attaching anonymous maps to the amp (anonymous map pointer) member in the segvn_data structure. Figure 9.11 shows a seg_vn segment with the anonymous map structures allocated to it.

Figure 9.11. Anonymous Memory Data Structures

Every allocated page-sized piece of virtual memory in the segment is assigned an anonymous map slot. For example, when a segment is first created, there are no anonymous map slots allocated, but the first time a zero fill-on-demand page fault occurs, a slot is allocated for a page, corresponding to the address within the segment where the fault occurred. At that time, a physical page is attached to the slot. Later, the page may be stolen and no page is associated anymore, but an empty slot remains.