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

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Date:   Tue, 14 May 2019 16:41:05 +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
Subject: Re: [PATCH RFC v2 0/4] mm/ksm: add option to automerge VMAs

[This is adding a new user visible interface so you should be CCing
linux-api mailing list. Also CC Hugh for KSM in general. Done now]

On Tue 14-05-19 15:16:50, Oleksandr Natalenko wrote:
> By default, KSM works only on memory that is marked by madvise(). And the
> only way to get around that is to either:
> 
>   * use LD_PRELOAD; or
>   * patch the kernel with something like UKSM or PKSM.
> 
> Instead, lets implement a sysfs knob, which allows marking VMAs as
> mergeable. This can be used manually on some task in question or by some
> small userspace helper daemon.
> 
> The knob is named "force_madvise", and it is write-only. It accepts a PID
> to act on. To mark the VMAs as mergeable, use:
> 
>    # echo PID > /sys/kernel/mm/ksm/force_madvise
> 
> To unmerge all the VMAs, use the same approach, prepending the PID with
> the "minus" sign:
> 
>    # echo -PID > /sys/kernel/mm/ksm/force_madvise
> 
> This patchset is based on earlier Timofey's submission [1], but it doesn't
> use dedicated kthread to walk through the list of tasks/VMAs. Instead,
> it is up to userspace to traverse all the tasks in /proc if needed.
> 
> The previous suggestion [2] was based on amending do_anonymous_page()
> handler to implement fully automatic mode, but this approach was
> incorrect due to improper locking and not desired due to excessive
> complexity.
> 
> The current approach just implements minimal interface and leaves the
> decision on how and when to act to userspace.

Please make sure to describe a usecase that warrants adding a new
interface we have to maintain for ever.

> 
> Thanks.
> 
> [1] https://lore.kernel.org/patchwork/patch/1012142/
> [2] http://lkml.iu.edu/hypermail/linux/kernel/1905.1/02417.html
> 
> Oleksandr Natalenko (4):
>   mm/ksm: introduce ksm_enter() helper
>   mm/ksm: introduce ksm_leave() helper
>   mm/ksm: introduce force_madvise knob
>   mm/ksm: add force merging/unmerging documentation
> 
>  Documentation/admin-guide/mm/ksm.rst |  11 ++
>  mm/ksm.c                             | 160 +++++++++++++++++++++------
>  2 files changed, 137 insertions(+), 34 deletions(-)
> 
> -- 
> 2.21.0
> 

-- 
Michal Hocko
SUSE Labs