linux-kernel - [RFC 5/8] mm: avoid using set_page_count() when pages are freed into allocator

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Message-Id: <20211026173822.502506-6-pasha.tatashin@soleen.com>
Date:   Tue, 26 Oct 2021 17:38:19 +0000
From:   Pasha Tatashin <pasha.tatashin@...een.com>
To:     pasha.tatashin@...een.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-m68k@...ts.linux-m68k.org,
        anshuman.khandual@....com, willy@...radead.org,
        akpm@...ux-foundation.org, william.kucharski@...cle.com,
        mike.kravetz@...cle.com, vbabka@...e.cz, geert@...ux-m68k.org,
        schmitzmic@...il.com, rostedt@...dmis.org, mingo@...hat.com,
        hannes@...xchg.org, guro@...com, songmuchun@...edance.com,
        weixugc@...gle.com, gthelen@...gle.com
Subject: [RFC 5/8] mm: avoid using set_page_count() when pages are freed into allocator

When struct pages are first initialized the page->_refcount field is
set 1. However, later when pages are freed into allocator we set
_refcount to 0 via set_page_count(). Unconditionally resetting
_refcount is dangerous.

Instead use page_ref_dec_return(), and verify that the _refcount is
what is expected.

Signed-off-by: Pasha Tatashin <pasha.tatashin@...een.com>
---
 mm/page_alloc.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 6af4596bddc2..9d18e5f9a85a 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1659,6 +1659,7 @@ void __free_pages_core(struct page *page, unsigned int order)
 	unsigned int nr_pages = 1 << order;
 	struct page *p = page;
 	unsigned int loop;
+	int refcnt;
 
 	/*
 	 * When initializing the memmap, __init_single_page() sets the refcount
@@ -1669,10 +1670,12 @@ void __free_pages_core(struct page *page, unsigned int order)
 	for (loop = 0; loop < (nr_pages - 1); loop++, p++) {
 		prefetchw(p + 1);
 		__ClearPageReserved(p);
-		set_page_count(p, 0);
+		refcnt = page_ref_dec_return(p);
+		VM_BUG_ON_PAGE(refcnt, p);
 	}
 	__ClearPageReserved(p);
-	set_page_count(p, 0);
+	refcnt = page_ref_dec_return(p);
+	VM_BUG_ON_PAGE(refcnt, p);
 
 	atomic_long_add(nr_pages, &page_zone(page)->managed_pages);
 
@@ -2244,10 +2247,12 @@ void __init init_cma_reserved_pageblock(struct page *page)
 {
 	unsigned i = pageblock_nr_pages;
 	struct page *p = page;
+	int refcnt;
 
 	do {
 		__ClearPageReserved(p);
-		set_page_count(p, 0);
+		refcnt = page_ref_dec_return(p);
+		VM_BUG_ON_PAGE(refcnt, p);
 	} while (++p, --i);
 
 	set_pageblock_migratetype(page, MIGRATE_CMA);
-- 
2.33.0.1079.g6e70778dc9-goog