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

[Andy Lutomirski]

On Wed, Jun 25, 2014 at 2:04 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Tue, Jun 24, 2014 at 6:40 PM, Ren, Qiaowei <qiaowei.ren@xxxxxxxxx> wrote:
>> On 2014-06-25, Andy Lutomirski wrote:
>>> On Mon, Jun 23, 2014 at 10:53 PM, Ren, Qiaowei <qiaowei.ren@xxxxxxxxx>
>>> 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.

Hmm.  the memfd_create thing may be able to do this for you.  If you
created a per-mm memfd and mapped it, it all just might work.

--Andy
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