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: <7a8eb8c0-35e5-4f45-bdd5-11a775ae752d@redhat.com>
Date: Fri, 19 Apr 2024 11:19:49 +0200
From: David Hildenbrand <david@...hat.com>
To: "Yin, Fengwei" <fengwei.yin@...el.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 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.

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!

-- 
Cheers,

David / dhildenb