Re: [PATCH v6 02/10] x86, mpx: add MPX specific mmap interface

[Andy Lutomirski <luto@amacapital.net>]

On Tue, Jun 24, 2014 at 6:40 PM, Ren, Qiaowei <qiaowei.ren@intel.com> wrote:
> On 2014-06-25, Andy Lutomirski wrote:
>> On Mon, Jun 23, 2014 at 10:53 PM, Ren, Qiaowei <qiaowei.ren@intel.com>
>> wrote:
>>> On 2014-06-24, Andy Lutomirski wrote:
>>>>> On 06/23/2014 01:06 PM, Andy Lutomirski wrote:
>>>>>> Can the new vm_operation "name" be use for this?  The magic
>>>>>> "always written to core dumps" feature might need to be reconsidered.
>>>>>
>>>>> One thing I'd like to avoid is an MPX vma getting merged with a
>>>>> non-MPX vma.  I don't see any code to prevent two VMAs with
>>>>> different vm_ops->names from getting merged.  That seems like a
>>>>> bit of a design oversight for ->name.  Right?
>>>>
>>>> AFAIK there are no ->name users that don't also set ->close, for
>>>> exactly that reason.  I'd be okay with adding a check for ->name, too.
>>>>
>>>> Hmm.  If MPX vmas had a real struct file attached, this would all
>>>> come for free. Maybe vmas with non-default vm_ops and file != NULL
>>>> should never be mergeable?
>>>>
>>>>>
>>>>> Thinking out loud a bit... There are also some more complicated
>>>>> but more performant cleanup mechanisms that I'd like to go after in the future.
>>>>> Given a page, we might want to figure out if it is an MPX page or not.
>>>>> I wonder if we'll ever collide with some other user of vm_ops->name.
>>>>> It looks fairly narrowly used at the moment, but would this keep
>>>>> us from putting these pages on, say, a tmpfs mount?  Doesn't look
>>>>> that way at the moment.
>>>>
>>>> You could always check the vm_ops pointer to see if it's MPX.
>>>>
>>>> One feature I've wanted: a way to have special per-process vmas that
>>>> can be easily found.  For example, I want to be able to efficiently
>>>> find out where the vdso and vvar vmas are.  I don't think this is
>>>> currently supported.
>>>>
>>> Andy, if you add a check for ->name to avoid the MPX vmas merged
>>> with
>> non-MPX vmas, I guess the work flow should be as follow (use
>> _install_special_mapping to get a new vma):
>>>
>>> unsigned long mpx_mmap(unsigned long len) {
>>>     ......
>>>     static struct vm_special_mapping mpx_mapping = {
>>>         .name = "[mpx]",
>>>         .pages = no_pages,
>>>     };
>>>
>>>     ....... vma = _install_special_mapping(mm, addr, len, vm_flags,
>>>     &mpx_mapping); ......
>>> }
>>>
>>> Then, we could check the ->name to see if the VMA is MPX specific. Right?
>>
>> Does this actually create a vma backed with real memory?  Doesn't this
>> need to go through anon_vma or something?  _install_special_mapping
>> completely prevents merging.
>>
> Hmm, _install_special_mapping should completely prevent merging, even among MPX vmas.
>
> So, could you tell me how to set MPX specific ->name to the vma when it is created? Seems like that I could not find such interface.

You may need to add one.

I'd suggest posting a new thread to linux-mm describing what you need
and asking how to do it.

--Andy