Specifications

This is the Title of the Book, eMatter Edition

The mmap Device Operation

427

To make these operations active for a specific mapping, it is necessary to store a

pointer to

simple_remap_vm_ops in the vm_ops field of the relevant VMA. This is usu-

ally done in the mmap method. If you turn back to the simple_remap_mmap exam-

ple, you see these lines of code:

vma->vm_ops = &simple_remap_vm_ops;

simple_vma_open(vma);

Note the explicit call to simple_vma_open. Since the open method is not invoked on

the initial mmap, we must call it explicitly if we want it to run.

Mapping Memory with nopage

Although remap_pfn_range works well for many, if not most, driver mmap imple-

mentations, sometimes it is necessary to be a little more flexible. In such situations,

an implementation using the nopage VMA method may be called for.

One situation in which the nopage approach is useful can be brought about by the

mremap system call, which is used by applications to change the bounding addresses

of a mapped region. As it happens, the kernel does not notify drivers directly when a

mapped VMA is changed by mremap. If the VMA is reduced in size, the kernel can

quietly flush out the unwanted pages without telling the driver. If, instead, the VMA

is expanded, the driver eventually finds out by way of calls to nopage when map-

pings must be set up for the new pages, so there is no need to perform a separate

notification. The nopage method, therefore, must be implemented if you want to

support the mremap system call. Here, we show a simple implementation of nopage

for the simple device.

The nopage method, remember, has the following prototype:

struct page *(*nopage)(struct vm_area_struct *vma,

unsigned long address, int *type);

When a user process attempts to access a page in a VMA that is not present in mem-

ory, the associated nopage function is called. The

address parameter contains the vir-

tual address that caused the fault, rounded down to the beginning of the page. The

nopage function must locate and return the

struct page pointer that refers to the

page the user wanted. This function must also take care to increment the usage

count for the page it returns by calling the get_page macro:

get_page(struct page *pageptr);

This step is necessary to keep the reference counts correct on the mapped pages. The

kernel maintains this count for every page; when the count goes to

0, the kernel

knows that the page may be placed on the free list. When a VMA is unmapped, the

kernel decrements the usage count for every page in the area. If your driver does not

increment the count when adding a page to the area, the usage count becomes

0 pre-

maturely, and the integrity of the system is compromised.

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