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Date:   Thu, 26 Oct 2017 16:16:04 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Fan Du <fan.du@...el.com>, akpm@...ux-foundation.org, hch@....de,
        dan.j.williams@...el.com, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org
Subject: Re: [PATCH 2/2] Add /proc/PID/{smaps, numa_maps} support for DAX

On Wed 25-10-17 10:14:34, Dave Hansen wrote:
> On 10/25/2017 02:30 AM, Michal Hocko wrote:
> >>
> >> 7f6c17800000-7f6c17e00000 rw-s 00000000 00:06 20559  /dev/dax12.0
> >> Size:               6144 kB
> >> .
> >> .
> >> .
> >> Ptes@2MB:      		6144 kB
> > This says how but it doesn't tell why and who is going to use the
> > information and what for.
> 
> The original motivation was for DAX.  They have parallel large page
> infrastructure separate from hugetlbfs and THP.  Their constraints about
> when they can use large pages differ from the normal mm cases, so it is
> hard to tell when large pages are in use.  For instance, the file on
> *disk* has to be 2MB contiguous and aligned to map with 2MB pages even
> if the mmap() address is >2MB and 2MB-aligned.

I assume there is some tool which is going to use this information?

> But, in general, this seems like the thing that we probably should have
> done in the first place for THP.  It's a lot more generic and does not
> require someone reading the file to know what the particular
> architecture's page sizes are.

I fully agree. This just shows how single usecase focus driven smaps
file was. That is why I am asking about usecases when somebody want to
try yet another special field there. Smaps has become a dump of of
special cases which is not really easy to understand and so people tend
to use it incorrectly.

-- 
Michal Hocko
SUSE Labs

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