linux-kernel - Re: [PATCH v6] mm/ksm: introduce ksm

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Date:   Thu, 12 May 2022 15:37:53 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Oleksandr Natalenko <oleksandr@...alenko.name>
Cc:     cgel.zte@...il.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, corbet@....net,
        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>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v6] mm/ksm: introduce ksm_force for each process

On Tue, 10 May 2022 15:30:36 +0200 Oleksandr Natalenko <oleksandr@...alenko.name> wrote:

> > If ksm_force is set to 1, force all anonymous and 'qualified' VMAs
> > of this mm to be involved in KSM scanning without explicitly calling
> > madvise to mark VMA as MADV_MERGEABLE. But It is effective only when
> > the klob of /sys/kernel/mm/ksm/run is set as 1.
> > 
> > If ksm_force is set to 0, cancel the feature of ksm_force of this
> > process (fallback to the default state) and unmerge those merged pages
> > belonging to VMAs which is not madvised as MADV_MERGEABLE of this process,
> > but still leave MADV_MERGEABLE areas merged.
> 
> To my best knowledge, last time a forcible KSM was discussed (see threads [1], [2], [3] and probably others) it was concluded that a) procfs was a horrible interface for things like this one; and b) process_madvise() syscall was among the best suggested places to implement this (which would require a more tricky handling from userspace, but still).
> 
> So, what changed since that discussion?
> 
> P.S. For now I do it via dedicated syscall, but I'm not trying to upstream this approach.

Why are you patching the kernel with a new syscall rather than using
process_madvise()?