Merge branch 'for-linus' of git://oss.sgi.com/xfs/xfs

[pandora-kernel.git] / Documentation / nommu-mmap.txt

diff --git a/Documentation/nommu-mmap.txt b/Documentation/nommu-mmap.txt
index 7714f57..b565e82 100644 (file)
--- a/Documentation/nommu-mmap.txt
+++ b/Documentation/nommu-mmap.txt
@@ -109,12 +109,18 @@ and it's also much more restricted in the latter case:
 FURTHER NOTES ON NO-MMU MMAP
 ============================
 
 FURTHER NOTES ON NO-MMU MMAP
 ============================
 

- (*) A request for a private mapping of less than a page in size may not return
-     a page-aligned buffer. This is because the kernel calls kmalloc() to
-     allocate the buffer, not get_free_page().
+ (*) A request for a private mapping of a file may return a buffer that is not
+     page-aligned.  This is because XIP may take place, and the data may not be
+     paged aligned in the backing store.

 
 

- (*) A list of all the mappings on the system is visible through /proc/maps in
-     no-MMU mode.
+ (*) A request for an anonymous mapping will always be page aligned.  If
+     possible the size of the request should be a power of two otherwise some
+     of the space may be wasted as the kernel must allocate a power-of-2
+     granule but will only discard the excess if appropriately configured as
+     this has an effect on fragmentation.
+
+ (*) A list of all the private copy and anonymous mappings on the system is
+     visible through /proc/maps in no-MMU mode.

 
  (*) A list of all the mappings in use by a process is visible through
      /proc/<pid>/maps in no-MMU mode.
 
  (*) A list of all the mappings in use by a process is visible through
      /proc/<pid>/maps in no-MMU mode.


@@ -242,3 +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.
 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'.