path: root/Documentation/nommu-mmap.txt
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Diffstat (limited to 'Documentation/nommu-mmap.txt')
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diff --git a/Documentation/nommu-mmap.txt b/Documentation/nommu-mmap.txt
index b565e8279d1..8e1ddec2c78 100644
--- a/Documentation/nommu-mmap.txt
+++ b/Documentation/nommu-mmap.txt
@@ -119,6 +119,32 @@ FURTHER NOTES ON NO-MMU MMAP
granule but will only discard the excess if appropriately configured as
this has an effect on fragmentation.
+ (*) The memory allocated by a request for an anonymous mapping will normally
+ be cleared by the kernel before being returned in accordance with the
+ Linux man pages (ver 2.22 or later).
+ In the MMU case this can be achieved with reasonable performance as
+ regions are backed by virtual pages, with the contents only being mapped
+ to cleared physical pages when a write happens on that specific page
+ (prior to which, the pages are effectively mapped to the global zero page
+ from which reads can take place). This spreads out the time it takes to
+ initialize the contents of a page - depending on the write-usage of the
+ mapping.
+ In the no-MMU case, however, anonymous mappings are backed by physical
+ pages, and the entire map is cleared at allocation time. This can cause
+ significant delays during a userspace malloc() as the C library does an
+ anonymous mapping and the kernel then does a memset for the entire map.
+ However, for memory that isn't required to be precleared - such as that
+ returned by malloc() - mmap() can take a MAP_UNINITIALIZED flag to
+ indicate to the kernel that it shouldn't bother clearing the memory before
+ returning it. Note that CONFIG_MMAP_ALLOW_UNINITIALIZED must be enabled
+ to permit this, otherwise the flag will be ignored.
+ uClibc uses this to speed up malloc(), and the ELF-FDPIC binfmt uses this
+ to allocate the brk and stack region.
(*) A list of all the private copy and anonymous mappings on the system is
visible through /proc/maps in no-MMU mode.