"In general, the kernel's memory allocator does not like to fail. So, when kernel code requests memory, the memory management code will work hard to satisfy the request. If this work involves pushing other pages out to swap or removing data from the page cache, so be it. A big exception happens, though, when an atomic allocation (using the GFP_ATOMIC flag) is requested. Code requesting atomic allocations is generally not in a position where it can wait around for a lot of memory housecleaning work; in particular, such code cannot sleep. So if the memory manager is unable to satisfy an atomic allocation with the memory it has in hand, it has no choice except to fail the request.
"Such failures are quite rare, especially when single pages are requested. The kernel works to keep some spare pages around at all times, so the memory stress must be severe before a single-page allocation will fail..."
