linux-kernel - Re: [PATCH 2/2] Add /proc/PID/{smaps, numa

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Date:   Thu, 26 Oct 2017 07:24:14 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Michal Hocko <mhocko@...nel.org>
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 10/26/2017 07:16 AM, Michal Hocko wrote:
>> 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?

Actually, I don't remember whether it was tooling or just confused
humans.  I *think* Dan was trying to write test cases for huge page DAX
support and couldn't figure out whether or not it was using large pages.

>> 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.

We just have to be careful not to cram use-cases into the existing
fields which might make them meaningless.  I fear that Fan Du's latest
patches do that.