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

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Date:   Wed, 15 May 2019 10:33:21 +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>
Subject: Re: [PATCH RFC v2 0/4] mm/ksm: add option to automerge VMAs

On Wed 15-05-19 09:37:23, Oleksandr Natalenko wrote:
[...]
> > This is way too generic. Please provide something more specific. Ideally
> > with numbers. Why those usecases cannot use an existing interfaces.
> > Remember you are trying to add a new user interface which we will have
> > to maintain for ever.
> 
> For my current setup with 2 Firefox instances I get 100 to 200 MiB saved
> for the second instance depending on the amount of tabs.

What does prevent Firefox (an opensource project) to be updated to use
the explicit merging?

[...]

> Answering your question regarding using existing interfaces, since
> there's only one, madvise(2), this requires modifying all the
> applications one wants to de-duplicate. In case of containers with
> arbitrary content or in case of binary-only apps this is pretty hard if
> not impossible to do properly.

OK, this makes more sense. Please note that there are other people who
would like to see certain madvise operations to be done on a remote
process - e.g. to allow external memory management (Android would like
to control memory aging so something like MADV_DONTNEED without loosing
content and more probably) and potentially other madvise operations.
Or maybe we need a completely new interface other than madvise.

In general, having a more generic API that would cover more usecases is
definitely much more preferable than one ad-hoc API that handles a very
specific usecase. So please try to think about a more generic
-- 
Michal Hocko
SUSE Labs