mm: optimize put_mems_allowed() usage

Since put_mems_allowed() is strictly optional, its a seqcount retry, we
don't need to evaluate the function if the allocation was in fact
successful, saving a smp_rmb some loads and comparisons on some relative
fast-paths.

Since the naming, get/put_mems_allowed() does suggest a mandatory
pairing, rename the interface, as suggested by Mel, to resemble the
seqcount interface.

This gives us: read_mems_allowed_begin() and read_mems_allowed_retry(),
where it is important to note that the return value of the latter call
is inverted from its previous incarnation.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index 7a13f6ac5421..068cd2a63d32 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -520,10 +520,10 @@ struct page *__page_cache_alloc(gfp_t gfp)
         if (cpuset_do_page_mem_spread()) {
                 unsigned int cpuset_mems_cookie;
                 do {
-                        cpuset_mems_cookie = get_mems_allowed();
+                        cpuset_mems_cookie = read_mems_allowed_begin();
                         n = cpuset_mem_spread_node();
                         page = alloc_pages_exact_node(n, gfp, 0);
-                } while (!put_mems_allowed(cpuset_mems_cookie) && !page);
+                } while (!page && read_mems_allowed_retry(cpuset_mems_cookie));
 
                 return page;
         }