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Date: Mon, 23 Oct 2017 10:49:11 +0200
From: Michal Hocko <mhocko@suse.com>
Subject: Re: [rfc 2/2] smaps: Show zone device memory used
Message-ID: <20171023084911.glsz6sd22mq2ey2o@dhcp22.suse.cz>
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To: Balbir Singh <bsingharora@gmail.com>
Cc: =?iso-8859-1?B?Suly9G1l?= Glisse <jglisse@redhat.com>, linux-mm <linux-mm@kvack.org>

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.
-- 
Michal Hocko
SUSE Labs

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