2 files changed, 29 insertions, 1 deletions

diff --git a/mm/Kconfig b/mm/Kconfig
index 57971d2ab848..c2b57d81e153 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -225,3 +225,31 @@ config HAVE_MLOCKED_PAGE_BIT
 
 config MMU_NOTIFIER
         bool
+
+config NOMMU_INITIAL_TRIM_EXCESS
+        int "Turn on mmap() excess space trimming before booting"
+        depends on !MMU
+        default 1
+        help
+          The NOMMU mmap() frequently needs to allocate large contiguous chunks
+          of memory on which to store mappings, but it can only ask the system
+          allocator for chunks in 2^N*PAGE_SIZE amounts - which is frequently
+          more than it requires.  To deal with this, mmap() is able to trim off
+          the excess and return it to the allocator.
+
+          If trimming is enabled, the excess is trimmed off and returned to the
+          system allocator, which can cause extra fragmentation, particularly
+          if there are a lot of transient processes.
+
+          If trimming is disabled, the excess is kept, but not used, which for
+          long-term mappings means that the space is wasted.
+
+          Trimming can be dynamically controlled through a sysctl option
+          (/proc/sys/vm/nr_trim_pages) which specifies the minimum number of
+          excess pages there must be before trimming should occur, or zero if
+          no trimming is to occur.
+
+          This option specifies the initial value of this option.  The default
+          of 1 says that all excess pages should be trimmed.
+
+          See Documentation/nommu-mmap.txt for more information.
diff --git a/mm/nommu.c b/mm/nommu.c
index 809998aa7b50..67cd1a487ee6 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -66,7 +66,7 @@ struct percpu_counter vm_committed_as;
 int sysctl_overcommit_memory = OVERCOMMIT_GUESS; /* heuristic overcommit */
 int sysctl_overcommit_ratio = 50; /* default is 50% */
 int sysctl_max_map_count = DEFAULT_MAX_MAP_COUNT;
-int sysctl_nr_trim_pages = 1; /* page trimming behaviour */
+int sysctl_nr_trim_pages = CONFIG_NOMMU_INITIAL_TRIM_EXCESS;
 int heap_stack_gap = 0;
 
 atomic_long_t mmap_pages_allocated;