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

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Date:   Tue, 13 Nov 2018 21:54:13 +0300
From:   Timofey Titovets <timofey.titovets@...esis.ru>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     Oleksandr Natalenko <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

вт, 13 нояб. 2018 г. в 21:35, Pavel Tatashin <pasha.tatashin@...een.com>:
>
> 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
>

That must work, but we out of bit space in vm_flags [1].
i.e. first 32 bits already defined, and other only accessible only on
64-bit machines.

1. https://elixir.bootlin.com/linux/latest/source/include/linux/mm.h#L219