linux-kernel - [PATCH v2] mm/rmap: do not add fully unmapped large folio to deferred split list

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Date: Wed, 24 Apr 2024 17:10:31 -0400
From: Zi Yan <zi.yan@...t.com>
To: linux-mm@...ck.org
Cc: Zi Yan <ziy@...dia.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Matthew Wilcox (Oracle)" <willy@...radead.org>,
	Yang Shi <shy828301@...il.com>,
	Ryan Roberts <ryan.roberts@....com>,
	Barry Song <21cnbao@...il.com>,
	David Hildenbrand <david@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH v2] mm/rmap: do not add fully unmapped large folio to deferred split list

From: Zi Yan <ziy@...dia.com>

In __folio_remove_rmap(), a large folio is added to deferred split list
if any page in a folio loses its final mapping. It is possible that
the folio is unmapped fully, but it is unnecessary to add the folio
to deferred split list at all. Fix it by checking folio->_nr_pages_mapped
before adding a folio to deferred split list. If the folio is already
on the deferred split list, it will be skipped.

Commit 98046944a159 ("mm: huge_memory: add the missing
folio_test_pmd_mappable() for THP split statistics") tried to exclude
mTHP deferred split stats from THP_DEFERRED_SPLIT_PAGE, but it does not
fix everything. A fully unmapped PTE-mapped order-9 THP was also added to
deferred split list and counted as THP_DEFERRED_SPLIT_PAGE, since nr is
512 (non zero), level is RMAP_LEVEL_PTE, and inside deferred_split_folio()
the order-9 folio is folio_test_pmd_mappable(). However, this miscount
was present even earlier due to implementation, since PTEs are unmapped
individually and first PTE unmapping adds the THP into the deferred split
list.

With commit b06dc281aa99 ("mm/rmap: introduce
folio_remove_rmap_[pte|ptes|pmd]()"), kernel is able to unmap PTE-mapped
folios in one shot without causing the miscount, hence this patch.

Signed-off-by: Zi Yan <ziy@...dia.com>
---
 mm/rmap.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index a7913a454028..220ad8a83589 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1553,9 +1553,11 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
 		 * page of the folio is unmapped and at least one page
 		 * is still mapped.
 		 */
-		if (folio_test_large(folio) && folio_test_anon(folio))
-			if (level == RMAP_LEVEL_PTE || nr < nr_pmdmapped)
-				deferred_split_folio(folio);
+		if (folio_test_large(folio) && folio_test_anon(folio) &&
+		    list_empty(&folio->_deferred_list) &&
+		    ((level == RMAP_LEVEL_PTE && atomic_read(mapped)) ||
+		     (level == RMAP_LEVEL_PMD && nr < nr_pmdmapped)))
+			deferred_split_folio(folio);
 	}

 	/*

base-commit: 2541ee5668b019c486dd3e815114130e35c1495d
-- 
2.43.0