linux-kernel - Re: [PATCH] [RFC] mm: mmap: Allow mmap(MAP

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Message-ID: <c4d0805b-127b-436d-900b-4742ddbc1d15@gmx.de>
Date: Thu, 12 Sep 2024 03:45:33 +0200
From: Helge Deller <deller@....de>
To: Yang Shi <shy828301@...il.com>
Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Helge Deller <deller@...nel.org>, linux-kernel@...r.kernel.org,
 Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
 linux-parisc@...r.kernel.org
Subject: Re: [PATCH] [RFC] mm: mmap: Allow mmap(MAP_STACK) to map growable
 stack

On 9/12/24 03:08, Yang Shi wrote:
> On Wed, Sep 11, 2024 at 5:50 PM Helge Deller <deller@....de> wrote:
>>
>> On 9/12/24 01:05, Liam R. Howlett wrote:
>>> * Yang Shi <shy828301@...il.com> [240911 18:16]:
>>>> On Wed, Sep 11, 2024 at 12:49 PM Liam R. Howlett
>>>> <Liam.Howlett@...cle.com> wrote:
>>>>>
>>>>> * Helge Deller <deller@...nel.org> [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@....de>
>>>>>>
>>>>>> 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?
>
> Different sources and reports showed having huge pages for stack
> mapping hurts performance. A lot of applications, for example, pthread
> lib, allocate stack with MAP_STACK and they don't call MADV_NOHUGEPAGE
> on stack mapping.

That's true, and my patch does not change the behaviour wrt huge pages.
Using MAP_STACK still tags the area VM_NOHUGEPAGE. See below...


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