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
