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

From: Helge Deller
Date: Wed Sep 11 2024 - 22:10:20 EST

On 9/12/24 03:32, Liam R. Howlett wrote:
* Helge Deller <deller@xxxxxx> [240911 20:51]:
On 9/12/24 01:05, Liam R. Howlett wrote:
* 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.

Wouldn't a plain mmap() followed by madvise(MADV_NOHUGEPAGE) do exactly that?
Why abusing MAP_STACK for that?

I can think of two answers:
1. An error that has worked without issues so far
2. One less system call

I'm not saying this really is a blocker, but the change is not without
risk as it does change behaviour the user could see.

Interestingly enough, the man page is incorrect as it is written because
the flag is not strictly a no-op; it ensures no huge pages. So the
feature of applying VM_NOHUGEPAGE with the use of MAP_STACK is not
documented today.

Yes.

What happens to call that use the mmap(MAP_GROWSDOWN | MAP_STACK) on
parisc today?

Today, without my patch, on parisc the area is then flagged VM_GROWSDOWN only.
In that case, stack expansion will not work, as it checks for VM_GROWSUP.

How does this change with your VM_STACK change? Wouldn't this result
in failed mappings?
VM_GROWSDOWN | VM_GROWSUP would fail in do_mmap(), and these would be> set if you map MAP_STACK to VM_STACK which is defined as VM_GROWSUP?

Right, with my change, the area will be tagged VM_GROWSUP and VM_GROWSDOWN.
Due to VM_GROWSUP stack expansion works.
The mapping doesn't fail in do_mmap(), because stacks are not file-mapped
or shared or droppable. They should be mapped with MAP_PRIVATE, right?


Another option is to introduce an alias, e.g.:
#define MAP_STACK_EXPANDABLE MAP_GROWSDOWN
and then
diff --git a/include/linux/mman.h b/include/linux/mman.h
index bcb201ab7a41..6a7ec3e0078a 100644
--- a/include/linux/mman.h
+++ b/include/linux/mman.h
@@ -153,7 +153,7 @@ calc_vm_prot_bits(unsigned long prot, unsigned long pkey)
static inline unsigned long
calc_vm_flag_bits(unsigned long flags)
{
- return _calc_vm_trans(flags, MAP_GROWSDOWN, VM_GROWSDOWN ) |
+ return _calc_vm_trans(flags, MAP_STACK_EXPANDABLE, VM_STACK ) |
_calc_vm_trans(flags, MAP_LOCKED, VM_LOCKED ) |
_calc_vm_trans(flags, MAP_SYNC, VM_SYNC ) |
_calc_vm_trans(flags, MAP_STACK, VM_NOHUGEPAGE) |

This simply uses the existing MAP_GROWSDOWN flag to mark a stack,
is independend on the stack growth direction and is fully backwards
compatible to existing behaviour.

I prefer my initial proposal to change MAP_STACK though, as it's
simpler and clearer.

Helge