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:

quoted

quoted

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