>-----Original Message----- >From: Michal Hocko [mailto:mho...@kernel.org] >Sent: Friday, October 27, 2017 4:43 PM >To: Du, Fan <fan...@intel.com> >Cc: Hansen, Dave <dave.han...@intel.com>; a...@linux-foundation.org; >h...@lst.de; Williams, Dan J <dan.j.willi...@intel.com>; >linux-kernel@vger.kernel.org >Subject: Re: [PATCH v4] Add /proc/PID/smaps support for DAX > >On Fri 27-10-17 08:24:07, Du, Fan wrote: >> >> >> >-----Original Message----- >> >From: Michal Hocko [mailto:mho...@kernel.org] >> >Sent: Friday, October 27, 2017 4:08 PM >> >To: Du, Fan <fan...@intel.com> >> >Cc: Hansen, Dave <dave.han...@intel.com>; a...@linux-foundation.org; >> >h...@lst.de; Williams, Dan J <dan.j.willi...@intel.com>; >> >linux-kernel@vger.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...@intel.com>; a...@linux-foundation.org; >h...@lst.de; >> >> >Williams, Dan J <dan.j.willi...@intel.com>; mho...@kernel.org >> >> >Cc: linux-kernel@vger.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! >> >> If user created device DAX, how did one know how much memory being >used? >> by "used" I mean, page table mapping created, fact is no way to find out >here. >> >> why does this master? The answer is same as I bought 512G SSD disk, why >> do I want to check disk usage with `df`?! if application use only 128G, it >makes >> no sense at all for me to buy more redundant persistent memory if Cloud >provider >> has more other option to choose. > >I am not deeply familiar with DAX but I would expect that most users >will use a FS on top of it where we have standard tools. If the use is >direct then I can see how this make things more complicated but smaps is >not the right answer IMHO. Why? Well, just consider that you map the >same portions of the device multiple times for whatever reason and you >are screwed because you have no means to distinguish those. So you will >get bogus numbers. Or am I misunderstanding something? > >If you need a DAX device statistics then make them device specific. This >is the only reliable way to get valid data.
Thanks for your comments, I will try to discuss with Dan about this. Have a nice day! >-- >Michal Hocko >SUSE Labs
