linux-kernel - Re: [PATCH v5 3/6] ksm: count all zero pages placed by KSM

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Date:   Thu, 9 Feb 2023 20:01:57 +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 3/6] ksm: count all zero pages placed by KSM

> > diff --git a/mm/ksm.c b/mm/ksm.c
> > index 652c088f9786..72c0722be280 100644
> > --- a/mm/ksm.c
> > +++ b/mm/ksm.c
> > @@ -276,6 +276,9 @@ static unsigned int zero_checksum __read_mostly;
> >   /* Whether to merge empty (zeroed) pages with actual zero pages */
> >   static bool ksm_use_zero_pages __read_mostly;
> > 
> > +/* The number of zero pages placed by KSM use_zero_pages */
> > +static unsigned long ksm_zero_pages_sharing;
> 
> Does this count how many zero pages are currently placed or how many 
> rmap items with ZERO_PAGE_FLAG are in the system?

Yes, it counts how many ksm zero pages are currently placed. and we use
rmap items with ZERO_PAGE_FLAG to record these zero page, similar to 
ksm_pages_sharing and ksm_pages_shared which are recorded by the rmap_items
with STABLE_FLAG.

> 
> IOW, if someone triggers MADV_DONTNEED on such a zeropage, the counter 
> will not get updated, correct?

Well, the counter can get updated as someone triggers MADV_DONTNEED on such
a zeropage. You might write a simple code to test it.