From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1757633AbcDGV3D (ORCPT <rfc822;w@1wt.eu>);
	Thu, 7 Apr 2016 17:29:03 -0400
Received: from g9t5008.houston.hp.com ([15.240.92.66]:46894 "EHLO
	g9t5008.houston.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1757219AbcDGV3B (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Thu, 7 Apr 2016 17:29:01 -0400
Message-ID: <1460064033.20338.74.camel@hpe.com>
Subject: Re: [PATCH] x86 get_unmapped_area: Add PMD alignment for DAX PMD
 mmap
From: Toshi Kani <toshi.kani@hpe.com>
To: Matthew Wilcox <willy@linux.intel.com>
Cc: mingo@kernel.org, bp@suse.de, hpa@zytor.com, tglx@linutronix.de,
        dan.j.williams@intel.com, kirill.shutemov@linux.intel.com,
        linux-mm@kvack.org, x86@kernel.org, linux-nvdimm@ml01.01.org,
        linux-kernel@vger.kernel.org
Date: Thu, 07 Apr 2016 15:20:33 -0600
In-Reply-To: <20160407174111.GG2781@linux.intel.com>
References: <1459951089-14911-1-git-send-email-toshi.kani@hpe.com>
	 <20160406165027.GA2781@linux.intel.com> <1459964672.20338.41.camel@hpe.com>
	 <20160407174111.GG2781@linux.intel.com>
Content-Type: text/plain; charset="UTF-8"
X-Mailer: Evolution 3.18.5.2 (3.18.5.2-1.fc23) 
Mime-Version: 1.0
Content-Transfer-Encoding: 8bit
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Thu, 2016-04-07 at 13:41 -0400, Matthew Wilcox wrote:
> On Wed, Apr 06, 2016 at 11:44:32AM -0600, Toshi Kani wrote:
> > > 
> > > The NVML chooses appropriate addresses and gets a properly aligned
> > > address without any kernel code.
> >
> > An application like NVML can continue to specify a specific address to
> > mmap().  Most existing applications, however, do not specify an address
> > to mmap().  With this patch, specifying an address will remain
> > optional.
>
> The point is that this *can* be done in userspace.  You need to sell us
> on the advantages of doing it in the kernel.

Sure.  As I said, the point is that we do not need to modify existing
applications for using DAX PMD mappings.

For instance, fio with "ioengine=mmap" performs I/Os with mmap().
https://github.com/caius/fio/blob/master/engines/mmap.c

With this change, unmodified fio can be used for testing with DAX PMD
mappings.  There are many examples like this, and I do not think we want to
modify all applications that we want to evaluate/test with.

> > > I think this is the wrong place for it, if we decide that this is the
> > > right thing to do.  The filesystem has a get_unmapped_area() which
> > > should be used instead.
> >
> > Yes, I considered adding a filesystem entry point, but decided going
> > this way because:
> >  - arch_get_unmapped_area() and arch_get_unmapped_area_topdown() are
> > arch-specific code.  Therefore, this filesystem entry point will need
> > arch-specific implementation. 
> >  - There is nothing filesystem specific about requesting PMD alignment.
>
> See http://article.gmane.org/gmane.linux.kernel.mm/149227 for Hugh's
> approach for shmem.  I strongly believe that if we're going to do this
> i the kernel, we should build on this approach, and not hack something
> into each architecture's generic get_unmapped_area.

Thanks for the pointer.  Yes, we can call current->mm->get_unmapped_area()
with size + PMD_SIZE, and adjust with the alignment in a filesystem entry
point.  I will update the patch with this approach.

-Toshi