Re: [rfc 2/2] smaps: Show zone device memory used

[Balbir Singh <bsingharora@gmail.com>]

On Sat, Oct 21, 2017 at 12:08 AM, Michal Hocko <mhocko@suse.com> wrote:
> On Wed 18-10-17 17:31:23, Balbir Singh wrote:
>> With HMM, we can have either public or private zone
>> device pages. With private zone device pages, they should
>> show up as swapped entities. For public zone device pages
>> the smaps output can be confusing and incomplete.
>>
>> This patch adds a new attribute to just smaps to show
>> device memory usage.
>
> As this will become user API which we will have to maintain for ever I
> would really like to hear about who is going to use this information and
> what for.

This is something I observed when running some tests with HMM/CDM.
The issue I had was that there was no visibility of what happened to the
pages after the following sequence

1. malloc/mmap pages
2. migrate_vma() to ZONE_DEVICE (hmm/cdm space)
3. look at smaps

If we look at smaps after 1 and the pages are faulted in we can see the
pages for the region, but at point 3, there is absolutely no visibility of
what happened to the pages. I thought smaps is a good way to provide
the visibility as most developers use that interface. It's more to fix the
inconsistency I saw w.r.t visibility and accounting.

Balbir Singh.

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