linux-kernel - Re: [PATCH RFC v2 0/4] mm/ksm: add option to automerge VMAs

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Message-ID: <20190515145151.GG16651@dhcp22.suse.cz>
Date:   Wed, 15 May 2019 16:51:51 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Oleksandr Natalenko <oleksandr@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Kirill Tkhai <ktkhai@...tuozzo.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Timofey Titovets <nefelim4ag@...il.com>,
        Aaron Tomlin <atomlin@...hat.com>,
        Grzegorz Halat <ghalat@...hat.com>, linux-mm@...ck.org,
        linux-api@...r.kernel.org, Hugh Dickins <hughd@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH RFC v2 0/4] mm/ksm: add option to automerge VMAs

[Cc Suren and Minchan - the email thread starts here 20190514131654.25463-1-oleksandr@...hat.com]

On Wed 15-05-19 08:53:11, Michal Hocko wrote:
[...]
> I will try to comment on the interface itself later. But I have to say
> that I am not impressed. Abusing sysfs for per process features is quite
> gross to be honest.

I have already commented on this in other email. I consider sysfs an
unsuitable interface for per-process API. Not to mention this particular
one is very KSM specific while the question about setting different
hints on memory of a remote process is a more generic question. As
already mentioned there are usecases where people would like to say
that a certain memory is cold from outside of the process context (e.g.
monitor application). So essentially a form of a user space memory
management. And this usecase sounds a bit similar to me and having a
common api sounds more sensible to me.

One thing we were discussing at LSFMM this year was a way to either
provide madvise_remote(pid, addr, length, advice) or a fadvise
alternative over /proc/<pid>/map_vmas/<range> file descriptors
(essentially resembling the existing map_files api) to achieve such a
functionality. This is still a very rough idea but the api would sound
much more generic to me and it would allow much wider range of usecases.

But maybe I am completely wrong and this is just opens a can of worms
that we do not want.
-- 
Michal Hocko
SUSE Labs