Linux adopts the smaller 4 KB page frame size as the standard memory allocation unit. This makes things simpler for two reasons:

1. The Page Fault exceptions issued by the paging circuitry are easily interpreted. Either the page requested exists but the process is not allowed to address it, or the page does not exist. In the second case, the memory allocator must find a free 4 KB page frame and assign it to the process.
   
   
2. Although both 4 KB and 4 MB are multiples of all disk block sizes, transfers of data between main memory and disks are in most cases more efficient when the smaller size is used.
   
   
3. The virt_to_page(addr) macro yields the address of the page descriptor associated with the linear address addr.
4. The pfn_to_page(pfn) macro yields the address of the page descriptor associated with the page frame having number pfn.
   
   

In the 80 x 86 UMA architecture the zones are: Contains page frames of memory at and above 16 MB and below 896 MB

• ZONE_HIGHMEM

Contains page frames of memory at and above 896 MB

• However, some kernel control paths cannot be blocked while requesting memorythis happens, for instance, when handling an interrupt or when executing code inside a critical region. In these cases, a kernel control path should issue atomic memory allocation requests (using the GFP_ATOMIC flag; see the later section "The Zoned Page Frame Allocator"). An atomic request never blocks: if there are not enough free pages, the allocation simply fails.
  
  
  
  

On 32-bit platforms such as the 80 x 86 architecture, however, Linux designers had to find some way to allow the kernel to exploit all the available RAM, up to the 64 GB supported by PAE. The approach adopted is the following:

• The allocation of high-memory page frames is done only through the alloc_pages( ) function and its alloc_page( ) shortcut. These functions do not return the linear address of the first allocated page frame, because if the page frame belongs to the high memory, such linear address simply does not exist. Instead, the functions return the linear address of the page descriptor of the first allocated page frame. These linear addresses always exist, because all page descriptors are allocated in low memory once and forever during the kernel initialization.
  
  
• Page frames in high memory that do not have a linear address cannot be accessed by the kernel. Therefore, part of the last 128 MB of the kernel linear address space is dedicated to mapping high-memory page frames. Of course, this kind of mapping is temporary, otherwise only 128 MB of high memory would be accessible. Instead, by recycling linear addresses the whole high memory can be accessed, although at different times.
  
  

The kernel uses three different mechanisms to map page frames in high memory; they are called permanent kernel mapping, temporary kernel mapping, and noncontiguous memory allocation.