mm: remove __GFP_COLD

As the page free path makes no distinction between cache hot and cold
pages, there is no real useful ordering of pages in the free list that
allocation requests can take advantage of.  Juding from the users of
__GFP_COLD, it is likely that a number of them are the result of copying
other sites instead of actually measuring the impact.  Remove the
__GFP_COLD parameter which simplifies a number of paths in the page
allocator.

This is potentially controversial but bear in mind that the size of the
per-cpu pagelists versus modern cache sizes means that the whole per-cpu
list can often fit in the L3 cache.  Hence, there is only a potential
benefit for microbenchmarks that alloc/free pages in a tight loop.  It's
even worse when THP is taken into account which has little or no chance
of getting a cache-hot page as the per-cpu list is bypassed and the
zeroing of multiple pages will thrash the cache anyway.

The truncate microbenchmarks are not shown as this patch affects the
allocation path and not the free path.  A page fault microbenchmark was
tested but it showed no sigificant difference which is not surprising
given that the __GFP_COLD branches are a miniscule percentage of the
fault path.

Link: http://lkml.kernel.org/r/20171018075952.10627-9-mgorman@techsingularity.net
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

4 files changed, 2 insertions, 16 deletions

diff --git a/include/linux/gfp.h b/include/linux/gfp.h
index f7e62d9096fe..1a4582b44d32 100644
--- a/include/linux/gfp.h
+++ b/include/linux/gfp.h
@@ -24,7 +24,6 @@ struct vm_area_struct;
 #define ___GFP_HIGH		0x20u
 #define ___GFP_IO		0x40u
 #define ___GFP_FS		0x80u
-#define ___GFP_COLD		0x100u
 #define ___GFP_NOWARN		0x200u
 #define ___GFP_RETRY_MAYFAIL	0x400u
 #define ___GFP_NOFAIL		0x800u
@@ -192,16 +191,12 @@ struct vm_area_struct;
 /*
  * Action modifiers
  *
- * __GFP_COLD indicates that the caller does not expect to be used in the near
- *   future. Where possible, a cache-cold page will be returned.
- *
  * __GFP_NOWARN suppresses allocation failure reports.
  *
  * __GFP_COMP address compound page metadata.
  *
  * __GFP_ZERO returns a zeroed page on success.
  */
-#define __GFP_COLD	((__force gfp_t)___GFP_COLD)
 #define __GFP_NOWARN	((__force gfp_t)___GFP_NOWARN)
 #define __GFP_COMP	((__force gfp_t)___GFP_COMP)
 #define __GFP_ZERO	((__force gfp_t)___GFP_ZERO)
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 4c6790bb7afb..34ce3ebf97d5 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -234,15 +234,9 @@ static inline struct page *page_cache_alloc(struct address_space *x)
 	return __page_cache_alloc(mapping_gfp_mask(x));
 }
 
-static inline struct page *page_cache_alloc_cold(struct address_space *x)
-{
-	return __page_cache_alloc(mapping_gfp_mask(x)|__GFP_COLD);
-}
-
 static inline gfp_t readahead_gfp_mask(struct address_space *x)
 {
-	return mapping_gfp_mask(x) |
-				  __GFP_COLD | __GFP_NORETRY | __GFP_NOWARN;
+	return mapping_gfp_mask(x) | __GFP_NORETRY | __GFP_NOWARN;
 }
 
 typedef int filler_t(void *, struct page *);
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index aa1341474916..7c46fd0b8b64 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -2672,7 +2672,7 @@ static inline struct page *__dev_alloc_pages(gfp_t gfp_mask,
 	 * 4.  __GFP_MEMALLOC is ignored if __GFP_NOMEMALLOC is set due to
 	 *     code in gfp_to_alloc_flags that should be enforcing this.
 	 */
-	gfp_mask |= __GFP_COLD | __GFP_COMP | __GFP_MEMALLOC;
+	gfp_mask |= __GFP_COMP | __GFP_MEMALLOC;
 
 	return alloc_pages_node(NUMA_NO_NODE, gfp_mask, order);
 }
diff --git a/include/linux/slab.h b/include/linux/slab.h
index 79f6532f8a0b..50697a1d6621 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -467,9 +467,6 @@ static __always_inline void *kmalloc_large(size_t size, gfp_t flags)
  * Also it is possible to set different flags by OR'ing
  * in one or more of the following additional @flags:
  *
- * %__GFP_COLD - Request cache-cold pages instead of
- *   trying to return cache-warm pages.
- *
  * %__GFP_HIGH - This allocation has high priority and may use emergency pools.
  *
  * %__GFP_NOFAIL - Indicate that this allocation is in no way allowed to fail