NOMMU: Make mmap allocation page trimming behaviour configurable.

NOMMU mmap allocates a piece of memory for an mmap that's rounded up in size to
the nearest power-of-2 number of pages.  Currently it then discards the excess
pages back to the page allocator, making that memory available for use by other
things.  This can, however, cause greater amount of fragmentation.

To counter this, a sysctl is added in order to fine-tune the trimming
behaviour.  The default behaviour remains to trim pages aggressively, while
this can either be disabled completely or set to a higher page-granular
watermark in order to have finer-grained control.

vm region vm_top bits taken from an earlier patch by David Howells.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Mike Frysinger <vapier.adi@gmail.com>

2 files changed, 33 insertions, 0 deletions

diff --git a/Documentation/nommu-mmap.txt b/Documentation/nommu-mmap.txt
index 02b89dc..b565e82 100644
--- a/Documentation/nommu-mmap.txt
+++ b/Documentation/nommu-mmap.txt
@@ -248,3 +248,18 @@ PROVIDING SHAREABLE BLOCK DEVICE SUPPORT
 Provision of shared mappings on block device files is exactly the same as for
 character devices. If there isn't a real device underneath, then the driver
 should allocate sufficient contiguous memory to honour any supported mapping.
+
+
+=================================
+ADJUSTING PAGE TRIMMING BEHAVIOUR
+=================================
+
+NOMMU mmap automatically rounds up to the nearest power-of-2 number of pages
+when performing an allocation.  This can have adverse effects on memory
+fragmentation, and as such, is left configurable.  The default behaviour is to
+aggressively trim allocations and discard any excess pages back in to the page
+allocator.  In order to retain finer-grained control over fragmentation, this
+behaviour can either be disabled completely, or bumped up to a higher page
+watermark where trimming begins.
+
+Page trimming behaviour is configurable via the sysctl `vm.nr_trim_pages'.
diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
index cd05994..a341507 100644
--- a/Documentation/sysctl/vm.txt
+++ b/Documentation/sysctl/vm.txt
@@ -38,6 +38,7 @@ Currently, these files are in /proc/sys/vm:
 - numa_zonelist_order
 - nr_hugepages
 - nr_overcommit_hugepages
+- nr_trim_pages		(only if CONFIG_MMU=n)
 
 ==============================================================
 
@@ -348,3 +349,20 @@ Change the maximum size of the hugepage pool. The maximum is
 nr_hugepages + nr_overcommit_hugepages.
 
 See Documentation/vm/hugetlbpage.txt
+
+==============================================================
+
+nr_trim_pages
+
+This is available only on NOMMU kernels.
+
+This value adjusts the excess page trimming behaviour of power-of-2 aligned
+NOMMU mmap allocations.
+
+A value of 0 disables trimming of allocations entirely, while a value of 1
+trims excess pages aggressively. Any value >= 1 acts as the watermark where
+trimming of allocations is initiated.
+
+The default value is 1.
+
+See Documentation/nommu-mmap.txt for more information.