linux-kernel - Re: [PATCH v4] Add /proc/PID/smaps support for DAX

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Message-ID: <20171027080730.3532fpgcwbozewfv@dhcp22.suse.cz>
Date:   Fri, 27 Oct 2017 10:07:30 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     "Du, Fan" <fan.du@...el.com>
Cc:     "Hansen, Dave" <dave.hansen@...el.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "hch@....de" <hch@....de>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4] Add /proc/PID/smaps support for DAX

On Fri 27-10-17 02:47:43, Du, Fan wrote:
> 
> 
> >-----Original Message-----
> >From: Hansen, Dave
> >Sent: Thursday, October 26, 2017 10:03 PM
> >To: Du, Fan <fan.du@...el.com>; akpm@...ux-foundation.org; hch@....de;
> >Williams, Dan J <dan.j.williams@...el.com>; mhocko@...nel.org
> >Cc: linux-kernel@...r.kernel.org
> >Subject: Re: [PATCH v4] Add /proc/PID/smaps support for DAX
> >
> >I'm honestly not understanding what problem this solves.  Could you,
> >perhaps, do a before and after of smaps with and without this patch?
> 
> The motivation here is described in the commit message.
> ------------------------------------------------------------------------------------------
> Memory behind device DAX is not attached into normal memory
> management system, when user mmap /dev/dax, smaps part is
> currently missing, so no idea for user to check how much
> device DAX memory are actually used in practice.

This might be motivation but you are still not explaining _why_ that is
a problem. _Who_ is going to use that information and for _what_
purpose. This is really essential!
-- 
Michal Hocko
SUSE Labs