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-rw-r--r--Documentation/nommu-mmap.txt31
1 files changed, 26 insertions, 5 deletions
diff --git a/Documentation/nommu-mmap.txt b/Documentation/nommu-mmap.txt
index 7714f57caad5..b565e8279d13 100644
--- 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
============================
- (*) 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.
@@ -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.
+
+
+=================================
+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'.