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

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Message-ID: <202302041418359766772@zte.com.cn>
Date:   Sat, 4 Feb 2023 14:18:35 +0800 (CST)
From:   <yang.yang29@....com.cn>
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>
Subject: Re: [PATCH v5 2/6] ksm: support unsharing zero pages placed by KSM

> 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).

> 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.