x86: add MAP_STACK mmap flag

as per this discussion:

   http://lkml.org/lkml/2008/8/12/423

Pardo reported that 64-bit threaded apps, if their stacks exceed the
combined size of ~4GB, slow down drastically in pthread_create() - because
glibc uses MAP_32BIT to allocate the stacks. The use of MAP_32BIT is
a legacy hack - to speed up context switching on certain early model
64-bit P4 CPUs.

So introduce a new flag to be used by glibc instead, to not constrain
64-bit apps like this.

glibc can switch to this new flag straight away - it will be ignored
by the kernel. If those old CPUs ever matter to anyone, support for
it can be implemented.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Ulrich Drepper <drepper@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/asm-x86/mman.h b/include/asm-x86/mman.h
index c1682b542daf..90bc4108a4fd 100644
--- a/include/asm-x86/mman.h
+++ b/include/asm-x86/mman.h
@@ -12,6 +12,7 @@
 #define MAP_NORESERVE   0x4000          /* don't check for reservations */
 #define MAP_POPULATE    0x8000          /* populate (prefault) pagetables */
 #define MAP_NONBLOCK    0x10000         /* do not block on IO */
+#define MAP_STACK       0x20000         /* give out an address that is best suited for process/thread stacks */
 
 #define MCL_CURRENT     1               /* lock all current mappings */
 #define MCL_FUTURE      2               /* lock all future mappings */