[PATCH] x86: add MAP_STACK mmap flag

[Ingo Molnar]

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@xxxxxxx>
---
 include/asm-x86/mman.h |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/include/asm-x86/mman.h b/include/asm-x86/mman.h
index c1682b5..e5852b5 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 32bit stack addresses on old CPUs */
 
 #define MCL_CURRENT	1		/* lock all current mappings */
 #define MCL_FUTURE	2		/* lock all future mappings */
--
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