The Linux Programming Interface

1042 Chapter 49

The pgoff and size arguments identify a file region whose position in memory is to

be changed. The pgoff argument specifies the start of the file region in units of the

system page size (as returned by sysconf(_SC_PAGESIZE)). The size argument speci-

fies the length of the file region, in bytes. The addr argument serves two purposes:

z It identifies the existing mapping whose pages we want to rearrange. In other

words, addr must be an address that falls somewhere within a region that was

previously mapped with mmap().

z It specifies the memory address at which the file pages identified by pgoff and

size are to be located.

Both addr and size should be specified as multiples of the system page size. If they

are not, they are rounded down to the nearest multiple of the page size.

Suppose that we use the following call to mmap() to map three pages of the

open file referred to by the descriptor fd, and that the call assigns the returned

address 0x4001a000 to addr:

ps = sysconf(_SC_PAGESIZE); /* Obtain system page size */

addr = mmap(0, 3 * ps, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

The following calls would then create the nonlinear mapping shown in Figure 49-5:

remap_file_pages(addr, ps, 0, 2, 0);

/* Maps page 0 of file into page 2 of region */

remap_file_pages(addr + 2 * ps, ps, 0, 0, 0);

/* Maps page 2 of file into page 0 of region */

Figure 49-5: A nonlinear file mapping

There are two other arguments to remap_file_pages() that we haven’t yet described:

z The prot argument is ignored, and must be specified as 0. In the future, it may

be possible to use this argument to change the protection of the memory

region affected by remap_file_pages(). In the current implementation, the pro-

tection remains the same as that on the entire VMA.

Virtual machines and garbage collectors are other applications that employ

multiple VMAs. Some of these applications need to be able to write-protect

Mapped file

(12288 bytes)

page 0 of mapping

maps page 2 of file

Memory region

(12288 bytes)

page 1 of mapping

maps page 1 of file

page 2 of mapping

maps page 0 of file

0x4001a000 0x4001b000 0x4001c000

Increasing virtual addresses

page 0 of file page 1 of file page 2 of file