linux-kernel - Re: [PATCH V3] KSM: allow dedup all tasks memory

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Date:   Tue, 13 Nov 2018 17:59:30 +0000
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Oleksandr Natalenko <oleksandr@...alenko.name>
Cc:     jannh@...gle.com, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        timofey.titovets@...esis.ru, willy@...radead.org
Subject: Re: [PATCH V3] KSM: allow dedup all tasks memory

On 18-11-13 15:23:50, Oleksandr Natalenko wrote:
> Hi.
> 
> > Yep. However, so far, it requires an application to explicitly opt in
> > to this behavior, so it's not all that bad. Your patch would remove
> > the requirement for application opt-in, which, in my opinion, makes
> > this way worse and reduces the number of applications for which this
> > is acceptable.
> 
> The default is to maintain the old behaviour, so unless the explicit
> decision is made by the administrator, no extra risk is imposed.

The new interface would be more tolerable if it honored MADV_UNMERGEABLE:

KSM default on: merge everything except when MADV_UNMERGEABLE is
excplicitly set.

KSM default off: merge only when MADV_MERGEABLE is set.

The proposed change won't honor MADV_UNMERGEABLE, meaning that
application programmers won't have a way to prevent sensitive data to be
every merged. So, I think, we should keep allow an explicit opt-out
option for applications.

> 
> > As far as I know, basically nobody is using KSM at this point. There
> > are blog posts from several cloud providers about these security risks
> > that explicitly state that they're not using memory deduplication.
> 
> I tend to disagree here. Based on both what my company does and what UKSM
> users do, memory dedup is a desired option (note "option" word here, not the
> default choice).

Lightweight containers is a use case for KSM: when many VMs share the
same small kernel. KSM is used in production by large cloud vendors.

Thank you,
Pasha