* Yang Shi <shy828301@xxxxxxxxx> [240911 18:16]: > On Wed, Sep 11, 2024 at 12:49 PM Liam R. Howlett > <Liam.Howlett@xxxxxxxxxx> wrote: > > > > * Helge Deller <deller@xxxxxxxxxx> [240911 15:20]: > > > 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 ) | > > > > Right now MAP_STACK can be used to set VM_NOHUGEPAGE, but this will > > change the user interface to create a vma that will grow. I'm not > > entirely sure this is okay? > > AFAICT, I don't see this is a problem. Currently huge page also skips > the VMAs with VM_GROWS* flags set. See vma_is_temporary_stack(). > __thp_vma_allowable_orders() returns 0 if the vma is a temporary > stack. If someone is using MAP_STACK to avoid having a huge page, they will also get a mapping that grows - which is different than what happens today. I'm not saying that's right, but someone could be abusing the existing flag and this will change the behaviour. > > > > > > > That is mmap(MAP_STACK) would set VM_NOHUGEPAGE right now, with this > > change you'd get VM_NOHUGEPAGE | VM_GROWS<something> > > > > > _calc_vm_trans(flags, MAP_STACK, VM_NOHUGEPAGE) | > > > arch_calc_vm_flag_bits(flags); > > > } > > > > >
