The following code snippet defines the watermark modifiers:

#define __GFP_ATOMIC ((__force gfp_t)___GFP_ATOMIC)
#define __GFP_HIGH ((__force gfp_t)___GFP_HIGH)
#define __GFP_MEMALLOC ((__force gfp_t)___GFP_MEMALLOC)
#define __GFP_NOMEMALLOC ((__force gfp_t)___GFP_NOMEMALLOC)

Following is list of watermark modifiers, which provide control over emergency reserve pools of memory:

• __GFP_ATOMIC: This flag indicates that allocation is high priority and the caller context cannot be put into wait.
• __GFP_HIGH: This flag indicates that the caller is high priority and granting allocation request is necessary for the system to make progress. Setting this flag will cause the allocator to access the emergency pool.
• __GFP_MEMALLOC: This flag allows access to all memory. This should only be used when the caller guarantees the allocation will allow more memory to be freed very shortly, for example, process exiting or swapping.
• __GFP_NOMEMALLOC: This flag is used to forbid access to all reserved emergency pools.