mm: page_alloc: hide some GFP internals and document the bits and flag combinations

Andrew stated the following

	We have quite a history of remote parts of the kernel using
	weird/wrong/inexplicable combinations of __GFP_ flags.	I tend
	to think that this is because we didn't adequately explain the
	interface.

	And I don't think that gfp.h really improved much in this area as
	a result of this patchset.  Could you go through it some time and
	decide if we've adequately documented all this stuff?

This patches first moves some GFP flag combinations that are part of the MM
internals to mm/internal.h. The rest of the patch documents the __GFP_FOO
bits under various headings and then documents the flag combinations. It
will not help callers that are brain damaged but the clarity might motivate
some fixes and avoid future mistakes.

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vitaly Wool <vitalywool@gmail.com>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 19 insertions, 0 deletions

diff --git a/mm/internal.h b/mm/internal.h
index ff0f1ad..5b7841f6 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -14,6 +14,25 @@
 #include <linux/fs.h>
 #include <linux/mm.h>
 
+/*
+ * The set of flags that only affect watermark checking and reclaim
+ * behaviour. This is used by the MM to obey the caller constraints
+ * about IO, FS and watermark checking while ignoring placement
+ * hints such as HIGHMEM usage.
+ */
+#define GFP_RECLAIM_MASK (__GFP_RECLAIM|__GFP_HIGH|__GFP_IO|__GFP_FS|\
+			__GFP_NOWARN|__GFP_REPEAT|__GFP_NOFAIL|\
+			__GFP_NORETRY|__GFP_MEMALLOC|__GFP_NOMEMALLOC)
+
+/* The GFP flags allowed during early boot */
+#define GFP_BOOT_MASK (__GFP_BITS_MASK & ~(__GFP_RECLAIM|__GFP_IO|__GFP_FS))
+
+/* Control allocation cpuset and node placement constraints */
+#define GFP_CONSTRAINT_MASK (__GFP_HARDWALL|__GFP_THISNODE)
+
+/* Do not use these with a slab allocator */
+#define GFP_SLAB_BUG_MASK (__GFP_DMA32|__GFP_HIGHMEM|~__GFP_BITS_MASK)
+
 void free_pgtables(struct mmu_gather *tlb, struct vm_area_struct *start_vma,
 		unsigned long floor, unsigned long ceiling);