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

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Message-ID: <17c68728-9125-4597-9e3e-764b209edcd7@intel.com>
Date: Fri, 19 Apr 2024 21:47:34 +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 4/19/2024 5:19 PM, David Hildenbrand wrote:
> On 19.04.24 04:29, Yin, Fengwei wrote:
>>
>>
>> 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.
> 
> folio_entire_mapcount() does not apply to small folios, so we must not 
> call that for small folios.
Right. I missed this part. Thanks for clarification.


Regards
Yin, Fengwei

> 
> Regarding hugetlb, subpage mapcounts are completely unused, except 
> subpage 0 mapcount, which is now *always* negative (storing a page type) 
> -- so there is no trusting on that value at all.
> 
> So in the end, it all looked cleanest when only special-casing on 
> partially-mappable folios where we know the entire mapcount exists and 
> we know that subapge mapcount 0 actually stores something reasonable 
> (not a type).
> 
> Thanks!
>