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

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Message-ID: <ca0ada59-e1a4-dd74-1a52-233c77b3ad4e@redhat.com>
Date:   Mon, 13 Feb 2023 13:44:07 +0100
From:   David Hildenbrand <david@...hat.com>
To:     yang.yang29@....com.cn
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
Subject: Re: [PATCH v5 2/6] ksm: support unsharing zero pages placed by KSM

On 04.02.23 07:18, yang.yang29@....com.cn wrote:

[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.

> 
>> 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?

-- 
Thanks,

David / dhildenb