From: Thomas Gleixner <tglx@linutronix.de>
To: Dave Hansen <dave.hansen@intel.com>
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 22:36:39 +0100 (CET) [thread overview]
Message-ID: <alpine.DEB.2.11.1411112233180.3935@nanos> (raw)
In-Reply-To: <54627512.7060806@intel.com>
On Tue, 11 Nov 2014, Dave Hansen wrote:
> 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.
I don't think it's a real issue.
> 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. :)
Indeed. Though I think that distangling specific parts of MPX from
mmap_sem is still a worthwhile exercise. So not all of the complex
ideas we came up with during the discussion are lost in the pointless
complexity universe :)
Thanks,
tglx
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next prev parent reply other threads:[~2014-11-11 21:36 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
2014-11-11 21:36 ` Thomas Gleixner [this message]
2014-10-12 4:41 ` [PATCH v9 12/12] x86, mpx: add documentation on Intel MPX Qiaowei Ren
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