linux-kernel - Re: [PATCH v3 2/4] pagemap: add mmap-exclusive bit for marking pages mapped only here

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Date:	Wed, 17 Jun 2015 08:11:51 +0000
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	Konstantin Khlebnikov <koct9i@...il.com>
CC:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
	Mark Williamson <mwilliamson@...o-software.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Kirill A. Shutemov" <kirill@...temov.name>
Subject: Re: [PATCH v3 2/4] pagemap: add mmap-exclusive bit for marking
 pages mapped only here

On Tue, Jun 09, 2015 at 11:00:17PM +0300, Konstantin Khlebnikov wrote:
> From: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
> 
> This patch sets bit 56 in pagemap if this page is mapped only once.
> It allows to detect exclusively used pages without exposing PFN:
> 
> present file exclusive state
> 0       0    0         non-present
> 1       1    0         file page mapped somewhere else
> 1       1    1         file page mapped only here
> 1       0    0         anon non-CoWed page (shared with parent/child)
> 1       0    1         anon CoWed page (or never forked)
> 
> CoWed pages in MAP_FILE|MAP_PRIVATE areas are anon in this context.
> 
> Mmap-exclusive bit doesn't reflect potential page-sharing via swapcache:
> page could be mapped once but has several swap-ptes which point to it.
> Application could detect that by swap bit in pagemap entry and touch
> that pte via /proc/pid/mem to get real information.
> 
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
> Link: http://lkml.kernel.org/r/CAEVpBa+_RyACkhODZrRvQLs80iy0sqpdrd0AaP_-tgnX3Y9yNQ@mail.gmail.com
> 
> ---
> 
> v2:
> * handle transparent huge pages
> * invert bit and rename shared -> exclusive (less confusing name)
> ---
...

> @@ -1119,6 +1122,13 @@ static int pagemap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
>  		else
>  			pmd_flags2 = 0;
>  
> +		if (pmd_present(*pmd)) {
> +			struct page *page = pmd_page(*pmd);
> +
> +			if (page_mapcount(page) == 1)
> +				pmd_flags2 |= __PM_MMAP_EXCLUSIVE;
> +		}
> +

Could you do the same thing for huge_pte_to_pagemap_entry(), too? 
                                                                  
Thanks,                                                           
Naoya Horiguchi                                                   --
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