docs/core-api: mm-api: add section about GFP flags

Link: http://lkml.kernel.org/r/1532626360-16650-8-git-send-email-rppt@linux.vnet.ibm.com
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/core-api/mm-api.rst

@@ -14,6 +14,27 @@ User Space Memory Access
 .. kernel-doc:: mm/util.c
    :functions: get_user_pages_fast
 
+Memory Allocation Controls
+==========================
+
+Functions which need to allocate memory often use GFP flags to express
+how that memory should be allocated. The GFP acronym stands for "get
+free pages", the underlying memory allocation function. Not every GFP
+flag is allowed to every function which may allocate memory. Most
+users will want to use a plain ``GFP_KERNEL``.
+
+.. kernel-doc:: include/linux/gfp.h
+   :doc: Page mobility and placement hints
+
+.. kernel-doc:: include/linux/gfp.h
+   :doc: Watermark modifiers
+
+.. kernel-doc:: include/linux/gfp.h
+   :doc: Reclaim modifiers
+
+.. kernel-doc:: include/linux/gfp.h
+   :doc: Common combinations
+
 The Slab Cache
 ==============