[PATCH 03/16] mm: slub: Optimise the SLUB fast path to avoid pfmemalloc checks

From: Christoph Lameter <redacted>

This patch removes the check for pfmemalloc from the alloc hotpath and
puts the logic after the election of a new per cpu slab. For a pfmemalloc
page we do not use the fast path but force the use of the slow path which
is also used for the debug case.

This has the side-effect of weakening pfmemalloc processing in the
following way;

1. A process that is allocating for network swap calls __slab_alloc.
   pfmemalloc_match is true so the freelist is loaded and c->freelist is
   now pointing to a pfmemalloc page.

2. A process that is attempting normal allocations calls slab_alloc,
   finds the pfmemalloc page on the freelist and uses it because it did
   not check pfmemalloc_match()

The patch allows non-pfmemalloc allocations to use pfmemalloc pages with
the kmalloc slabs being the most vunerable caches on the grounds they
are most likely to have a mix of pfmemalloc and !pfmemalloc requests. A
later patch will still protect the system as processes will get throttled
if the pfmemalloc reserves get depleted but performance will not degrade
as smoothly.

[mgorman@suse.de: Expanded changelog]
Signed-off-by: Christoph Lameter <redacted>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 mm/slub.c |    7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index f0909bf..f8cbec4 100644
--- a/mm/slub.c
+++ b/mm/slub.c

@@ -2298,11 +2298,11 @@ new_slab:
 		}
 	}
 
-	if (likely(!kmem_cache_debug(s)))
+	if (likely(!kmem_cache_debug(s) && pfmemalloc_match(c, gfpflags)))
 		goto load_freelist;
 
 	/* Only entered in the debug case */
-	if (!alloc_debug_processing(s, c->page, object, addr))
+	if (kmem_cache_debug(s) && !alloc_debug_processing(s, c->page, object, addr))
 		goto new_slab;	/* Slab failed checks. Next slab needed */
 
 	c->freelist = get_freepointer(s, object);

@@ -2352,8 +2352,7 @@ redo:
 	barrier();
 
 	object = c->freelist;
-	if (unlikely(!object || !node_match(c, node) ||
-					!pfmemalloc_match(c, gfpflags)))
+	if (unlikely(!object || !node_match(c, node)))
 		object = __slab_alloc(s, gfpflags, node, addr, c);
 
 	else {

-- 
1.7.9.2

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