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Date:   Tue, 04 Nov 2014 08:00:25 -0800
From:   Dave Hansen <dave.hansen@intel.com>
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To:     Thomas Gleixner <tglx@linutronix.de>
CC:     Ren Qiaowei <qiaowei.ren@intel.com>,
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Subject: Re: [PATCH v9 11/12] x86, mpx: cleanup unused bound tables
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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.