Re: [PATCH] [RFC] mm: mmap: Allow mmap(MAP_STACK) to map growable stack

[Liam R. Howlett]

* 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);
> > >  }
> > >
> >