Page allocator and other allocator routines (which we'll discuss in the following sections) need the gfp_mask flag as an argument, which is of type gfp_t:

typedef unsigned __bitwise__ gfp_t;

Gfp flags are used to supply two vital attributes for the allocator functions: the first is the mode of the allocation, which controls the behavior of the allocator function, and the second is the source of the allocation, which indicates the zone or list of zones from which memory can be sourced. The kernel header gfp.h defines various flag constants that are categorized into distinct groups, called zone modifiers, mobility and placement flags, watermark modifiers, reclaim modifiers, and action modifiers.