Re: [PATCH v9 11/12] x86, mpx: cleanup unused bound tables

Dave Hansen
2014-11-04T16:00:25Z

On 11/03/2014 01:29 PM, Thomas Gleixner wrote:
> On Mon, 3 Nov 2014, Dave Hansen wrote:

> That's not really true. You can evaluate that information with
> mmap_sem held for read as well. Nothing can change the mappings until
> you drop it. So you could do:
>
> down_write(mm->bd_sem);
> down_read(mm->mmap_sem;
> evaluate_size_of_shm_to_unmap();
> clear_bounds_directory_entries();
> up_read(mm->mmap_sem);
> do_the_real_shm_unmap();
> up_write(mm->bd_sem);
>
> That should still be covered by the above scheme.

Yep, that'll work. It just means rewriting the shmdt()/mremap() code to
do a "dry run" of sorts.

Do you have any concerns about adding another mutex to these paths?
munmap() isn't as hot of a path as the allocation side, but it does
worry me a bit that we're going to perturb some workloads. We might
need to find a way to optimize out the bd_sem activity on processes that
never used MPX.