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: <3ef81fe3-2584-4db8-ab66-eaa44c035707@redhat.com>
Date: Tue, 16 Apr 2024 12:47:07 +0200
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <ioworker0@...il.com>
Cc: akpm@...ux-foundation.org, cgroups@...r.kernel.org, chris@...kel.net,
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Subject: Re: [PATCH v1 05/18] mm: improve folio_likely_mapped_shared() using
 the mapcount of large folios

On 16.04.24 12:40, Lance Yang wrote:
> Hey David,
> 
> Maybe I spotted a bug below.

Thanks for the review!

> 
> [...]
>   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;
> +
> +	/* 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;
> 
> bug: if a PMD-mapped THP is exclusively mapped, the folio_entire_mapcount()
> function will return 1 (atomic_read(&folio->_entire_mapcount) + 1).

If it's exclusively mapped, then folio_mapcount(folio)==1. In which case 
the previous statement:

if (mapcount <= 1)
	return false;

Catches it.

IOW, once we reach this point we now that folio_mapcount(folio) > 1, and 
there must be something else besides the entire mapping ("more than once").


Or did I not address your concern?

-- 
Cheers,

David / dhildenb