linux-kernel - Re: [PATCH] mm/ksm: introduce ksm

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Message-ID: <YoyfqAApe+RyUyGk@dhcp22.suse.cz>
Date:   Tue, 24 May 2022 11:04:40 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     CGEL <cgel.zte@...il.com>
Cc:     Balbir Singh <bsingharora@...il.com>, akpm@...ux-foundation.org,
        ammarfaizi2@...weeb.org, oleksandr@...alenko.name,
        willy@...radead.org, linux-mm@...ck.org, corbet@....net,
        linux-kernel@...r.kernel.org, xu xin <xu.xin16@....com.cn>,
        Yang Yang <yang.yang29@....com.cn>,
        Ran Xiaokai <ran.xiaokai@....com.cn>,
        wangyong <wang.yong12@....com.cn>,
        Yunkai Zhang <zhang.yunkai@....com.cn>,
        Jiang Xuexin <jiang.xuexin@....com.cn>
Subject: Re: [PATCH] mm/ksm: introduce ksm_enabled for each processg

On Tue 24-05-22 08:52:02, CGEL wrote:
> On Thu, May 19, 2022 at 09:39:57AM +0200, Michal Hocko wrote:
> > On Thu 19-05-22 06:35:03, CGEL wrote:
> > > On Wed, May 18, 2022 at 02:14:28PM +0200, Michal Hocko wrote:
> > > > On Wed 18-05-22 07:40:30, CGEL wrote:
> > > > [...]
> > > > > 2. process_madvise is still a kind of madvise. processs_madvise from
> > > > > another process overrides the intention of origin app code ifself that
> > > > > also calls madvise, which is unrecoverable. For example, if a process "A"
> > > > > which madvises just one part of VMAs (not all) as MERGEABLE run on the OS
> > > > > already, meanwhile, if another process which doesn't know the information
> > > > > of "A" 's MERGEABLE areas, then call process_madvise to advise all VMAs of
> > > > > "A" as MERGEABLE, the original MERGEABLE information of "A" calling madivse
> > > > > is erasured permanently.
> > > > 
> > > > I do not really follow. How is this any different from an external
> > > > process modifying the process wide policy via the proc or any other
> > > > interface?
> > > 
> > > In this patch, you can see that we didn't modify the flag of any VMA of
> > > the target process, which is different from process_madvise. So it is
> > > easy to keep the original MERGEABLE information of the target process
> > > when we turn back to the default state from the state "always".
> > 
> > This means that /proc/<pid>/smaps doesn't show the real state, right?
> 
> Maybe we can add extra information of KSM forcible state in /proc/<pid>/smaps
> like THPeligible. 

That information is already printed and I do not think that adding
another flag or whatever would make the situation much more clear.

> Really, Michal, I think it again, 'process_ madvise' is really not good. In
> addition to some shortcomings I said before, If new vmas of the target process
> are created after the external process calls process_madvise(), then we have to
> call `process_madvise()` on them again, over and over again, regularly, just like
> Oleksandr said [1].

I can see that this is not the most convenient way but so far I haven't
really heard any arguments that this would be impossible.

Look, I am not claiming that process_madvise is the only way to achieve
the goal. I really do not like the proc based interface because it is
rather adhoc and limited. We have other means to set a process wide
property and I do not see any strong arguments agaist prctl.

But more importantly I haven't really seen any serious analysis whether
per-process (resp. per MM) property is even a desirable interface.
Especially in the current form when opting out for certain VMAs is not
possible.
 
> [1]: https://lore.kernel.org/lkml/1817008.tdWV9SEqCh@natalenko.name/

-- 
Michal Hocko
SUSE Labs