linux-kernel - Re: [PATCH v5 2/6] ksm: support unsharing zero pages placed by KSM

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Date:   Sat, 11 Mar 2023 05:37:14 +0000
From:   xu xin <xu.xin.sc@...il.com>
To:     david@...hat.com
Cc:     akpm@...ux-foundation.org, imbrenda@...ux.ibm.com,
        jiang.xuexin@....com.cn, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, ran.xiaokai@....com.cn, xu.xin.sc@...il.com,
        xu.xin16@....com.cn, yang.yang29@....com.cn
Subject: Re: [PATCH v5 2/6] ksm: support unsharing zero pages placed by KSM

[sorry to reply so late, on vacation too, and my mailing system has some kind of problem]

>[sorry, was on vacation last week]

>> Why use flags if they both conditions are mutually exclusive?
> 
> Just to make the return value of break_ksm_pmd_entry() more expressive and
> understandable. because break_ksm_pmd_entry have three types of returned
> values (0, 1, 2).

> It adds confusion. Just simplify it please.

So I think it's good to add a enum value of 0 listed here as suggested
by Claudio Imbrenda.

> 
>> MADV_UNMERGEABLE -> unmerge_ksm_pages() will never unshare the shared
>> zeropage? I thought the patch description mentions that that is one of
>> the goals?
> 
> No, MADV_UNMERGEABLE will trigger KSM to unshare the shared zeropages in the
> context of "get_next_rmap_item() -> unshare_zero_pages(), but not directly in the
> context of " madvise()-> unmerge_ksm_pages() ". The reason for this is to avoid
> increasing long delays of madvise() calling on unsharing zero pages.
> 

>Why do we care and make this case special?

Yeah, the code seems a bit special, but it is a helpless way and best choice, because the
action of unsharing zero-pages is too complex and CPU consuming because checking whether the
page we get is actually placed by KSM or not is not a easy thing in the context of
unmerge_ksm_pages.

In experiment, unsharing zero-pages in the context of unmerge_ksm_pages cause user' madvise()
spend 5 times the time than the way of the current patch.

So let's leave it as it is now. I will add a (short) explanation of when and why the new
unshare_zero_page flag should be used.

Sincerely.
Xu Xin