pd_data_t is the base element for a node
quoted from : Professional Linux kernel architecture by Wolfgang Mauerer, p 136-138.
when I studied kernel memory management, pg_data_t is the first data structure.
1. to distinguish between NUMA and UMA systems.
2. RAM is divided into nodes which are represented by pg_data.
3. Each node is split into zones
http://lxr.linux.no/#linux+v2.6.34/include/linux/mmzone.h#L631
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quoted from :
http://www.kernel.org/doc/man-pages/online/pages/man2/syscalls.2.html
The system call is the fundamental interface between an application and the Linux kernel.
System calls and library wrapper functions System calls are generally not invoked directly, but rather via wrapper functions in glibc (or perhaps some other library).
For details of direct invocation of a system call, see intro(2).
Often, but not always, the name of
the wrapper function is the same as the name of the system call that it invokes.
For example, glibc contains a function truncate() which invokes the underlying "truncate " system call.
Often the glibc wrapper function is quite thin, doing little work other than copying arguments to the right registers before invoking the system call, and then setting errno appropriately after the system call has returned.
( These are the same steps that are performed by syscall(2), which can be used to invoke system calls for which no wrapper function is provided.)
Note: system calls indicate a failure by returning a negative error number to the caller; when this happens, the wrapper function negates the returned error number (to make it positive), copies it to errno, and returns -1 to the caller of the
wrapper.
Sometimes, however, the wrapper function does some extra work before invoking the system call.
For example, nowadays there are (for reasons described below) two related system calls, truncate(2) and truncate64(2),
and the glibc
truncate() wrapper function checks which of those system calls are provided by the kernel and determines which should be employed.
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