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

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Date: Tue, 16 Apr 2024 18:40:08 +0800
From: Lance Yang <ioworker0@...il.com>
To: david@...hat.com
Cc: akpm@...ux-foundation.org,
	cgroups@...r.kernel.org,
	chris@...kel.net,
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	Lance Yang <ioworker0@...il.com>
Subject: Re: [PATCH v1 05/18] mm: improve folio_likely_mapped_shared() using the mapcount of large folios

Hey David,

Maybe I spotted a bug below.

[...]
 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).

IIUC, when mapping a PMD entry for the entire THP, folio->_entire_mapcount
increments from -1 to 0.

Thanks,
Lance

+
+	/* Let's guess based on the first subpage. */
+	return atomic_read(&folio->_mapcount) > 0;
 }
[...]