linux-kernel - Re: [PATCH v1 05/18] mm: improve folio_likely_mapped_shared() using the mapcount of large folios

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Date: Fri, 19 Apr 2024 10:29:44 +0800
From: "Yin, Fengwei" <fengwei.yin@...el.com>
To: David Hildenbrand <david@...hat.com>, <linux-kernel@...r.kernel.org>
CC: <linux-mm@...ck.org>, <linux-doc@...r.kernel.org>,
	<cgroups@...r.kernel.org>, <linux-sh@...r.kernel.org>,
	<linux-trace-kernel@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>, Andrew
 Morton <akpm@...ux-foundation.org>, "Matthew Wilcox (Oracle)"
	<willy@...radead.org>, Peter Xu <peterx@...hat.com>, Ryan Roberts
	<ryan.roberts@....com>, Yang Shi <shy828301@...il.com>, Zi Yan
	<ziy@...dia.com>, Jonathan Corbet <corbet@....net>, Hugh Dickins
	<hughd@...gle.com>, Yoshinori Sato <ysato@...rs.sourceforge.jp>, Rich Felker
	<dalias@...c.org>, John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
	Chris Zankel <chris@...kel.net>, Max Filippov <jcmvbkbc@...il.com>, "Muchun
 Song" <muchun.song@...ux.dev>, Miaohe Lin <linmiaohe@...wei.com>, "Naoya
 Horiguchi" <naoya.horiguchi@....com>, Richard Chang <richardycc@...gle.com>
Subject: Re: [PATCH v1 05/18] mm: improve folio_likely_mapped_shared() using
 the mapcount of large folios



On 4/10/2024 3:22 AM, David Hildenbrand wrote:
> @@ -2200,7 +2200,22 @@ static inline size_t folio_size(struct folio *folio)
>    */
>   static inline bool folio_likely_mapped_shared(struct folio *folio)
>   {
> -	return page_mapcount(folio_page(folio, 0)) > 1;
> +	int mapcount = folio_mapcount(folio);
> +
> +	/* Only partially-mappable folios require more care. */
> +	if (!folio_test_large(folio) || unlikely(folio_test_hugetlb(folio)))
> +		return mapcount > 1;
My understanding is that mapcount > folio_nr_pages(folio) can cover
order 0 folio. And also folio_entire_mapcount() can cover hugetlb (I am
not 100% sure for this one).  I am wondering whether we can drop above
two lines? Thanks.


Regards
Yin, Fengwei

> +
> +	/* A single mapping implies "mapped exclusively". */
> +	if (mapcount <= 1)
> +		return false;
> +
> +	/* If any page is mapped more than once we treat it "mapped shared". */
> +	if (folio_entire_mapcount(folio) || mapcount > folio_nr_pages(folio))
> +		return true;
> +
> +	/* Let's guess based on the first subpage. */
> +	return atomic_read(&folio->_mapcount) > 0;
>   }