From: Dave Hansen <dave.hansen@intel.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Qiaowei Ren <qiaowei.ren@intel.com>,
"H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
x86@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org,
linux-ia64@vger.kernel.org, linux-mips@linux-mips.org
Subject: Re: [PATCH v9 11/12] x86, mpx: cleanup unused bound tables
Date: Tue, 11 Nov 2014 12:44:02 -0800 [thread overview]
Message-ID: <54627512.7060806@intel.com> (raw)
In-Reply-To: <alpine.DEB.2.11.1411111213450.3935@nanos>
On 11/11/2014 10:27 AM, Thomas Gleixner wrote:
> On Thu, 6 Nov 2014, Dave Hansen wrote:
>> Instead of all of these games with dropping and reacquiring mmap_sem and
>> adding other locks, or deferring the work, why don't we just do a
>> get_user_pages()? Something along the lines of:
>>
>> while (1) {
>> ret = cmpxchg(addr)
>> if (!ret)
>> break;
>> if (ret == -EFAULT)
>> get_user_pages(addr);
>> }
>>
>> Does anybody see a problem with that?
>
> You want to do that under mmap_sem write held, right? Not a problem per
> se, except that you block normal faults for a possibly long time when
> the page(s) need to be swapped in.
Yeah, it might hold mmap_sem for write while doing this in the unmap
path. But, that's only if the bounds directory entry has been swapped
out. There's only 1 pointer of bounds directory entries there for every
1MB of data, so it _should_ be relatively rare. It would mean that
nobody's been accessing a 512MB swath of data controlled by the same
page of the bounds directory.
If it gets to be an issue, we can always add some code to fault it in
before mmap_sem is acquired.
FWIW, I believe we have a fairly long road ahead of us to optimize MPX
in practice. I have a list of things I want to go investigate, but I
have not looked in to it in detail at all.
> But yes, this might solve most of the issues at hand. Did not think
> about GUP at all :(
Whew. Fixing it was getting nasty and complicated. :)
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next prev parent reply other threads:[~2014-11-11 20:44 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-10-12 4:41 [PATCH v9 00/12] Intel MPX support Qiaowei Ren
2014-10-12 4:41 ` [PATCH v9 01/12] x86, mpx: introduce VM_MPX to indicate that a VMA is MPX specific Qiaowei Ren
2014-10-12 4:41 ` [PATCH v9 02/12] x86, mpx: rename cfg_reg_u and status_reg Qiaowei Ren
2014-10-12 4:41 ` [PATCH v9 03/12] x86, mpx: add MPX specific mmap interface Qiaowei Ren
2014-10-12 4:41 ` [PATCH v9 04/12] x86, mpx: add MPX to disaabled features Qiaowei Ren
2014-10-12 4:41 ` [PATCH v9 05/12] x86, mpx: on-demand kernel allocation of bounds tables Qiaowei Ren
2014-10-24 12:08 ` Thomas Gleixner
2014-10-27 3:20 ` Ren Qiaowei
2014-10-28 17:43 ` Dave Hansen
2014-10-28 17:57 ` Thomas Gleixner
2014-10-12 4:41 ` [PATCH v9 06/12] mpx: extend siginfo structure to include bound violation information Qiaowei Ren
2014-10-12 4:41 ` [PATCH v9 07/12] mips: sync struct siginfo with general version Qiaowei Ren
2014-10-12 4:41 ` [PATCH v9 08/12] ia64: " Qiaowei Ren
2014-10-12 4:41 ` [PATCH v9 09/12] x86, mpx: decode MPX instruction to get bound violation information Qiaowei Ren
2014-10-24 12:36 ` Thomas Gleixner
2014-10-27 1:43 ` Ren, Qiaowei
2014-10-27 20:36 ` Thomas Gleixner
2014-10-28 5:58 ` Ren Qiaowei
2014-10-31 20:16 ` Dave Hansen
2014-10-31 20:33 ` Thomas Gleixner
2014-10-30 22:38 ` Dave Hansen
2014-10-31 2:12 ` Ren Qiaowei
2014-10-31 9:09 ` Thomas Gleixner
2014-10-12 4:41 ` [PATCH v9 10/12] x86, mpx: add prctl commands PR_MPX_ENABLE_MANAGEMENT, PR_MPX_DISABLE_MANAGEMENT Qiaowei Ren
2014-10-24 12:49 ` Thomas Gleixner
2014-10-24 15:10 ` Thomas Gleixner
2014-10-27 2:17 ` Ren, Qiaowei
2014-10-27 20:38 ` Thomas Gleixner
2014-10-28 5:57 ` Ren Qiaowei
2014-10-12 4:41 ` [PATCH v9 11/12] x86, mpx: cleanup unused bound tables Qiaowei Ren
2014-10-24 14:40 ` Thomas Gleixner
2014-10-27 3:13 ` Ren Qiaowei
2014-10-27 20:49 ` Thomas Gleixner
2014-10-28 5:56 ` Ren Qiaowei
2014-10-28 10:42 ` Thomas Gleixner
2014-11-03 20:53 ` Dave Hansen
2014-11-03 21:29 ` Thomas Gleixner
2014-11-04 16:00 ` Dave Hansen
2014-11-04 17:02 ` Thomas Gleixner
2014-11-06 21:50 ` Dave Hansen
2014-11-11 18:27 ` Thomas Gleixner
2014-11-11 20:44 ` Dave Hansen [this message]
2014-11-11 21:36 ` Thomas Gleixner
2014-10-12 4:41 ` [PATCH v9 12/12] x86, mpx: add documentation on Intel MPX Qiaowei Ren
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