linux-kernel - [PATCH v1 5/6] mm/rmap: simplify PageAnonExclusive sanity checks when adding anon rmap

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Date:   Wed, 13 Sep 2023 14:51:12 +0200
From:   David Hildenbrand <david@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <muchun.song@...ux.dev>
Subject: [PATCH v1 5/6] mm/rmap: simplify PageAnonExclusive sanity checks when adding anon rmap

Let's sanity-check PageAnonExclusive vs. mapcount in page_add_anon_rmap()
and hugepage_add_anon_rmap() after setting PageAnonExclusive simply by
re-reading the mapcounts.

We can stop initializing the "first" variable in page_add_anon_rmap()
and no longer need an atomic_inc_and_test() in hugepage_add_anon_rmap().

While at it, switch to VM_WARN_ON_FOLIO().

Signed-off-by: David Hildenbrand <david@...hat.com>
---
 mm/rmap.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 489c142d073b..10d477a0991f 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1199,7 +1199,7 @@ void page_add_anon_rmap(struct page *page, struct vm_area_struct *vma,
 	atomic_t *mapped = &folio->_nr_pages_mapped;
 	int nr = 0, nr_pmdmapped = 0;
 	bool compound = flags & RMAP_COMPOUND;
-	bool first = true;
+	bool first;
 
 	/* Is page being mapped by PTE? Is this its first map to be added? */
 	if (likely(!compound)) {
@@ -1228,9 +1228,6 @@ void page_add_anon_rmap(struct page *page, struct vm_area_struct *vma,
 		}
 	}
 
-	VM_BUG_ON_PAGE(!first && (flags & RMAP_EXCLUSIVE), page);
-	VM_BUG_ON_PAGE(!first && PageAnonExclusive(page), page);
-
 	if (nr_pmdmapped)
 		__lruvec_stat_mod_folio(folio, NR_ANON_THPS, nr_pmdmapped);
 	if (nr)
@@ -1252,6 +1249,8 @@ void page_add_anon_rmap(struct page *page, struct vm_area_struct *vma,
 	}
 	if (flags & RMAP_EXCLUSIVE)
 		SetPageAnonExclusive(page);
+	VM_WARN_ON_FOLIO(page_mapcount(page) > 1 && PageAnonExclusive(page),
+			 folio);
 
 	mlock_vma_folio(folio, vma, compound);
 }
@@ -2532,15 +2531,14 @@ void hugepage_add_anon_rmap(struct page *page, struct vm_area_struct *vma,
 			    unsigned long address, rmap_t flags)
 {
 	struct folio *folio = page_folio(page);
-	int first;
 
 	VM_WARN_ON_FOLIO(!folio_test_anon(folio), folio);
 
-	first = atomic_inc_and_test(&folio->_entire_mapcount);
-	VM_BUG_ON_PAGE(!first && (flags & RMAP_EXCLUSIVE), page);
-	VM_BUG_ON_PAGE(!first && PageAnonExclusive(page), page);
+	atomic_inc(&folio->_entire_mapcount);
 	if (flags & RMAP_EXCLUSIVE)
 		SetPageAnonExclusive(page);
+	VM_WARN_ON_FOLIO(folio_entire_mapcount(folio) > 1 &&
+			 PageAnonExclusive(page), folio);
 }
 
 void hugepage_add_new_anon_rmap(struct folio *folio,
-- 
2.41.0