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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 794
SEC. 10.4
slab is available, it looks through the list of empty slabs. Finally, if necessary, it
will allocate a new slab, place the new task structure there, and link this slab with
the task-structure object cache. The
kernel service, which allocates physi-
cally contiguous memory regions in the kernel address space, is in fact built on top
of the slab and object cache interface described here.
A third memory allocator,
, is also available and is used when the re-
quested memory need be contiguous only in virtual space, not in physical memory.
In practice, this is true for most of the requested memory. One exception consists
of devices, which live on the other side of the memory bus and the memory man-
agement unit, and therefore do not understand virtual addresses. However, the use
results in some performance degradation, and it is used primarily for
allocating large amounts of contiguous virtual address space, such as for dynam-
ically inserting kernel modules. All these memory allocators are derived from
those in System V.
Virtual Address-Space Representation
The virtual address space is divided into homogeneous, contiguous, page-
aligned areas or regions. That is to say, each area consists of a run of consecutive
pages with the same protection and paging properties. The text segment and map-
ped files are examples of areas (see Fig. 10-13). There can be holes in the virtual
address space between the areas. Any memory reference to a hole results in a fatal
page fault. The page size is fixed, for example, 4 KB for the Pentium and 8 KB for
the Alpha. Starting with the Pentium, support for page frames of 4 MB was added.
On recent 64-bit architectures, Linux can support
huge pages
of 2 MB or 1 GB
each. In addition, in a
Physical Address Extension
) mode, which is used on
certain 32-bit architectures to increase the process address space beyond 4 GB,
page sizes of 2 MB are supported.
Each area is described in the kernel by a
entry. All the
s for a process are linked together in a list sorted on virtual address
so that all the pages can be found. When the list gets too long (more than 32 en-
tries), a tree is created to speed up searching it. The
entry lists the
area’s properties. These properties include the protection mode (e.g., read only or
read/write), whether it is pinned in memory (not pageable), and which direction it
grows in (up for data segments, down for stacks).
also records whether the area is private to the process or
shared with one or more other processes. After a
, Linux makes a copy of the
area list for the child process, but sets up the parent and child to point to the same
page tables. The areas are marked as read/write, but the pages themselves are
marked as read only.
If either process tries to write on a page, a protection fault
occurs and the kernel sees that the area is logically writable but the page is not
writeable, so it gives the process a copy of the page and marks it read/write. This
mechanism is how copy on write is implemented.

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