+ (*) 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.
+
git.openpandora.org / pandora-kernel.git / blobdiff