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

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Message-ID: <628c9cb4.1c69fb81.aec05.30a1@mx.google.com>
Date:   Tue, 24 May 2022 08:52:02 +0000
From:   CGEL <cgel.zte@...il.com>
To:     Michal Hocko <mhocko@...e.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 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. 

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].

[1]: https://lore.kernel.org/lkml/1817008.tdWV9SEqCh@natalenko.name/


> 
> -- 
> Michal Hocko
> SUSE Labs