This is a RFC to change the behaviour of mmap(MAP_STACK) to be sufficient to map memory for usage as stack on all architectures. Currently MAP_STACK is a no-op on Linux, and instead MAP_GROWSDOWN has to be used. To clarify, here is the relevant info from the mmap() man page: MAP_GROWSDOWN This flag is used for stacks. It indicates to the kernel virtual memory system that the mapping should extend downward in memory. The return address is one page lower than the memory area that is actually created in the process's virtual address space. Touching an address in the "guard" page below the mapping will cause the mapping to grow by a page. This growth can be repeated until the mapping grows to within a page of the high end of the next lower mapping, at which point touching the "guard" page will result in a SIGSEGV signal. MAP_STACK (since Linux 2.6.27) Allocate the mapping at an address suitable for a process or thread stack. This flag is currently a no-op on Linux. However, by employing this flag, applications can ensure that they transparently obtain support if the flag is implemented in the future. Thus, it is used in the glibc threading implementation to allow for the fact that some architectures may (later) require special treatment for stack allocations. A further reason to employ this flag is portability: MAP_STACK exists (and has an effect) on some other systems (e.g., some of the BSDs). The reason to suggest this change is, that on the parisc architecture the stack grows upwards. As such, using solely the MAP_GROWSDOWN flag will not work. Note that there exists no MAP_GROWSUP flag. By changing the behaviour of MAP_STACK to mark the memory area with the VM_STACK bit (which is VM_GROWSUP or VM_GROWSDOWN depending on the architecture) the MAP_STACK flag does exactly what people would expect on all platforms. This change should have no negative side-effect, as all code which used mmap(MAP_GROWSDOWN | MAP_STACK) still work as before. Signed-off-by: Helge Deller <deller@xxxxxx> diff --git a/include/linux/mman.h b/include/linux/mman.h index bcb201ab7a41..66bc72a0cb19 100644 --- a/include/linux/mman.h +++ b/include/linux/mman.h @@ -156,6 +156,7 @@ calc_vm_flag_bits(unsigned long flags) return _calc_vm_trans(flags, MAP_GROWSDOWN, VM_GROWSDOWN ) | _calc_vm_trans(flags, MAP_LOCKED, VM_LOCKED ) | _calc_vm_trans(flags, MAP_SYNC, VM_SYNC ) | + _calc_vm_trans(flags, MAP_STACK, VM_STACK ) | _calc_vm_trans(flags, MAP_STACK, VM_NOHUGEPAGE) | arch_calc_vm_flag_bits(flags); }
