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

[Ren, Qiaowei]

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?

Thanks,
Qiaowei