After this documentation was released in July 2003, I was approached by Prentice Hall and asked to write a book on the Linux VM under the Bruce Peren's Open Book Series.

The book is available and called simply "Understanding The Linux Virtual Memory Manager". There is a lot of additional material in the book that is not available here, including details on later 2.4 kernels, introductions to 2.6, a whole new chapter on the shared memory filesystem, coverage of TLB management, a lot more code commentary, countless other additions and clarifications and a CD with lots of cool stuff on it. This material (although now dated and lacking in comparison to the book) will remain available although I obviously encourge you to buy the book from your favourite book store :-) . As the book is under the Bruce Perens Open Book Series, it will be available 90 days after appearing on the book shelves which means it is not available right now. When it is available, it will be downloadable from http://www.phptr.com/perens so check there for more information.

To be fully clear, this webpage is not the actual book.

8.2 Allocating A Non-Contiguous Area

The functions vmalloc(), vmalloc_dma() and vmalloc_32() are provided to allocate a memory area that is contiguous in virtual address space. They all take a single parameter size which is rounded up to the next page alignment. They all return a linear address for the new allocated area.

Figure 8.2: Call Graph: vmalloc()

As is clear from the call graph shown in Figure 8.2, there are two steps to allocating the area.

The first step with get_vm_area() finds a region large enough to store the request. It searches through a linear linked list of vm_structs and returns a new struct describing the allocated region.

The second step is to allocate the necessary PGD entries with vmalloc_area_pages(), PMD entries with alloc_area_pmd() and PTE entries with alloc_area_pte() before finally allocating the page with alloc_page().

The page table updated by vmalloc() is not the current process but the master page table referenced by init_mm$\rightarrow$pgd. This means that a process accessing the vmalloc area will cause a page fault exception as its page tables are not pointing to the correct area. There is a special case in the page fault handling code which knows that the fault occured in the vmalloc area and updates the current process page tables using information from the master page table.

Table 8.1: Non-Contiguous Memory Allocation API

Table 8.2: Non-Contiguous Memory Free API