Chapter 1. Memory management

An UVM object — or also known as uobj — is a contiguous region of virtual memory backed by a specific system facility. This can be a file (vnode), XXX What else?.

In order to understand what "to be backed by" means, here is a review of some basic concepts of virtual memory management. In a system with virtual memory support, the system can manage an address space bigger than the physical amount of memory available to it. The address space is broken into chunks of fixed size, namely pages, as is the physical memory, which is divided into page frames.

When the system needs to access a memory address, it can either find the page it belongs to (page hit) or not (page fault). In the former case, the page is already stored in main memory so its data can be directly accessed. In the latter case, the page is not present in main memory.

When a page fault occurs, the processor's memory management unit (MMU) signals the kernel through an exception and asks it to handle the fault: this can either result in a resolved page fault or in an error. Assuming that all memory accesses are correct (and hence there are no errors), the kernel needs to bring the requested page into memory. But where is the requested page? Is it in the swap space? In a file? Should it be filled with zeros?

Here is where the backing mechanism enters the game. A backing object defines where the pages should be read from and where shall them be stored after modifications, if any. Talking about implementation, reading a page from the backing object is preformed by a getpages function while writing to it is done by a putpages one.

Example: consider a 32-bit address space, a page size of 4096 bytes and an uobj of 40960 bytes (10 pages) starting at the virtual address 0x00010000; this uobj's backing object is a vnode that represents a text file in your file system. Assume that the file has not been read at all yet, so none of its pages are in main memory. Now, the user requests a read from offset 5000 and with a length of 4000. This offset falls into the uobj's second page and the ending address (9000) falls into the third page. The kernel converts these logical offsets into memory addresses (0x00011388 and 0x00012328) and reads all the data contained in between. So what happens? The MMU causes two page faults and the vnode's getpages method is called for each of them, which then reads the pages from the corresponding file, puts them into main memory and returns control to the caller. At this point, the read has been served.

Similarly, pages can be modified in memory after they have been brought to it; at some point, these changes will need to be flushed to the backing store, which happens with the backing object's putpages operation. There are multiple reasons for the flush, including the need to reclaim the least recently used page frame from main memory, explicitly synchronizing the uobj with its backing store (think about synchronizing a file system), closing a file, etc.

Malloc types are used to group different allocation blocks into logical clusters so that the kernel can manage them in a more efficient manner.

A malloc type can be defined in a static or dynamic fashion. Types are defined statically when they are embedded in a piece of code that is linked together the kernel during build time; if they are part of a standalone module, they are defined dynamically.

For static declarations, the MALLOC_DEFINE(9) macro is provided, which is then used somewhere in the global scope of a source file. It has the following signature:

The first parameter takes the name of the malloc type to be defined; do not let the type shown above confuse you, because it is an internal detail you ought not know. Malloc types are often named in uppercase, prefixed by M_. Some examples include M_TEMP for temporary data, M_SOFTINTR for soft-interrupt structures, etc.

The second and third parameters are a character string describing the type; the former is a short description while the later provides a longer one.

For a dynamic declaration, you must first define the type as static within the source file. Later on, the malloc_type_attach(9) and malloc_type_detach(9) functions are used to notify the kernel about the presence or removal of the type; this is usually done in the module's initialization and finalization routines, respectively.