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

[Michal Hocko <mhocko@suse.com>]

On Mon 23-10-17 20:32:25, Balbir Singh wrote:
> On Mon, Oct 23, 2017 at 7:49 PM, Michal Hocko <mhocko@suse.com> wrote:
> > On Sat 21-10-17 08:55:59, Balbir Singh wrote:
> >> 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.
> >
> > Yes I can see how this can be confusing. But, well, I have grown overly
> > cautious regarding user APIs over time. So I would rather not add
> > something new until we have a real user with a usecase in mind. We can
> > always add this later but once we have exposed the accounting we are
> > bound to maintain it for ever.
> 
> I see your point. But we are beginning to build on top of this. I'll just add
> it as a private patch in my patchset. But soon, we'll need to address HMM/CDM
> pages of different sizes as well. My problem right now is to ensure correctness
> of the design and expectations and a large part of it is tracking
> where the pages are.

I would wait for more experiences to carve a userspace API into stone.
Adding one for debugging purposes is almost _always_ a bad idea that
back fires sooner or later.
-- 
Michal Hocko
SUSE Labs

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