path: root/Documentation/nommu-mmap.txt
diff options
authorJie Zhang <jie.zhang@analog.com>2009-12-14 18:00:02 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2009-12-15 08:53:24 -0800
commitea637639591def87a54cea811cbac796980cb30d (patch)
tree7ea3e4baf2ffade539ae30192521d331f8e863fa /Documentation/nommu-mmap.txt
parent5dc37642cbce34619e4588a9f0bdad1d2f870956 (diff)
nommu: fix malloc performance by adding uninitialized flag
The NOMMU code currently clears all anonymous mmapped memory. While this is what we want in the default case, all memory allocation from userspace under NOMMU has to go through this interface, including malloc() which is allowed to return uninitialized memory. This can easily be a significant performance penalty. So for constrained embedded systems were security is irrelevant, allow people to avoid clearing memory unnecessarily. This also alters the ELF-FDPIC binfmt such that it obtains uninitialised memory for the brk and stack region. Signed-off-by: Jie Zhang <jie.zhang@analog.com> Signed-off-by: Robin Getz <rgetz@blackfin.uclinux.org> Signed-off-by: Mike Frysinger <vapier@gentoo.org> Signed-off-by: David Howells <dhowells@redhat.com> Acked-by: Paul Mundt <lethal@linux-sh.org> Acked-by: Greg Ungerer <gerg@snapgear.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'Documentation/nommu-mmap.txt')
1 files changed, 26 insertions, 0 deletions
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.