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

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Date:   Tue, 13 Nov 2018 18:35:10 +0000
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Timofey Titovets <timofey.titovets@...esis.ru>
Cc:     oleksandr@...alenko.name, Jann Horn <jannh@...gle.com>,
        linux-doc@...r.kernel.org,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        linux-mm@...ck.org, Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH V3] KSM: allow dedup all tasks memory

On 18-11-13 21:17:42, Timofey Titovets wrote:
> вт, 13 нояб. 2018 г. в 20:59, Pavel Tatashin <pasha.tatashin@...een.com>:
> >
> > 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.
> >
> 
> We just did not have VM/Madvise flag for that currently.
> Same as THP.
> Because all logic written with assumption, what we have exactly 2 states.
> Allow/Disallow (More like not allow).
> 
> And if we try to add, that must be something like:
> MADV_FORBID_* to disallow something completely.

No need to add new user flag MADV_FORBID, we should keep MADV_MERGEABLE
and MADV_UNMERGEABLE, but make them work so when MADV_UNMERGEABLE is
set, memory is indeed becomes always unmergeable regardless of ksm mode
of operation.

To do the above in ksm_madvise(), a new state should be added, for example
instead of: 

case MADV_UNMERGEABLE:
	*vm_flags &= ~VM_MERGEABLE;

A new flag should be used:
	*vm_flags |= VM_UNMERGEABLE;

I think, without honoring MADV_UNMERGEABLE correctly, this patch won't
be accepted.

Pasha