linux-kernel - Re: [PATCH v6 02/10] x86, mpx: add MPX specific mmap interface

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Date:	Mon, 23 Jun 2014 14:04:22 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Dave Hansen <dave.hansen@...el.com>
Cc:	Qiaowei Ren <qiaowei.ren@...el.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH v6 02/10] x86, mpx: add MPX specific mmap interface

On Mon, Jun 23, 2014 at 1:28 PM, Dave Hansen <dave.hansen@...el.com> 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
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