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

[Ren, Qiaowei]

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.

Thanks,
Qiaowei
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