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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 <>
Signed-off-by: Robin Getz <>
Signed-off-by: Mike Frysinger <>
Signed-off-by: David Howells <>
Acked-by: Paul Mundt <>
Acked-by: Greg Ungerer <>
Signed-off-by: Andrew Morton <>
Signed-off-by: Linus Torvalds <>
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1 parent 5dc3764 commit ea637639591def87a54cea811cbac796980cb30d Jie Zhang committed with torvalds Dec 15, 2009
Showing with 60 additions and 4 deletions.
  1. +26 −0 Documentation/nommu-mmap.txt
  2. +2 −1 fs/binfmt_elf_fdpic.c
  3. +5 −0 include/asm-generic/mman-common.h
  4. +22 −0 init/Kconfig
  5. +5 −3 mm/nommu.c
@@ -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.
@@ -380,7 +380,8 @@ static int load_elf_fdpic_binary(struct linux_binprm *bprm,
current->mm->start_brk = do_mmap(NULL, 0, stack_size,
if (IS_ERR_VALUE(current->mm->start_brk)) {
@@ -19,6 +19,11 @@
#define MAP_TYPE 0x0f /* Mask for type of mapping */
#define MAP_FIXED 0x10 /* Interpret addr exactly */
#define MAP_ANONYMOUS 0x20 /* don't use a file */
+# define MAP_UNINITIALIZED 0x4000000 /* For anonymous mmap, memory could be uninitialized */
+# define MAP_UNINITIALIZED 0x0 /* Don't support this flag */
#define MS_ASYNC 1 /* sync memory asynchronously */
#define MS_INVALIDATE 2 /* invalidate the caches */
@@ -1079,6 +1079,28 @@ config SLOB
+ bool "Allow mmapped anonymous memory to be uninitialized"
+ depends on EMBEDDED && !MMU
+ default n
+ help
+ Normally, and according to the Linux spec, anonymous memory obtained
+ from mmap() has it's contents cleared before it is passed to
+ userspace. Enabling this config option allows you to request that
+ mmap() skip that if it is given an MAP_UNINITIALIZED flag, thus
+ providing a huge performance boost. If this option is not enabled,
+ then the flag will be ignored.
+ This is taken advantage of by uClibc's malloc(), and also by
+ ELF-FDPIC binfmt's brk and stack allocator.
+ Because of the obvious security issues, this option should only be
+ enabled on embedded devices where you control what is run in
+ userspace. Since that isn't generally a problem on no-MMU systems,
+ it is normally safe to say Y here.
+ See Documentation/nommu-mmap.txt for more information.
bool "Profiling support (EXPERIMENTAL)"
@@ -1143,9 +1143,6 @@ static int do_mmap_private(struct vm_area_struct *vma,
if (ret < rlen)
memset(base + ret, 0, rlen - ret);
- } else {
- /* if it's an anonymous mapping, then just clear it */
- memset(base, 0, rlen);
return 0;
@@ -1343,6 +1340,11 @@ unsigned long do_mmap_pgoff(struct file *file,
goto error_just_free;
+ /* clear anonymous mappings that don't ask for uninitialized data */
+ if (!vma->vm_file && !(flags & MAP_UNINITIALIZED))
+ memset((void *)region->vm_start, 0,
+ region->vm_end - region->vm_start);
/* okay... we have a mapping; now we have to register it */
result = vma->vm_start;

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