[PATCH 05/10] mm: avoid using set_page_count() when pages are freed into allocator

From: Pasha Tatashin <pasha.tatashin@soleen.com>
To: pasha.tatashin@soleen.com, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org, linux-m68k@lists.linux-m68k.org,
	anshuman.khandual@arm.com, willy@infradead.org,
	akpm@linux-foundation.org, william.kucharski@oracle.com,
	mike.kravetz@oracle.com, vbabka@suse.cz, geert@linux-m68k.org,
	schmitzmic@gmail.com, rostedt@goodmis.org, mingo@redhat.com,
	hannes@cmpxchg.org, guro@fb.com, songmuchun@bytedance.com,
	weixugc@google.com, gthelen@google.com, rientjes@google.com,
	pjt@google.com
Subject: [PATCH 05/10] mm: avoid using set_page_count() when pages are freed into allocator
Date: Wed,  8 Dec 2021 20:35:39 +0000	[thread overview]
Message-ID: <20211208203544.2297121-6-pasha.tatashin@soleen.com> (raw)
In-Reply-To: <20211208203544.2297121-1-pasha.tatashin@soleen.com>

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@soleen.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 b5554767b9de..13d989d62012 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1667,6 +1667,7 @@ 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
@@ -1677,10 +1678,12 @@ 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);
 
@@ -2252,10 +2255,12 @@ 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);
-- 
2.34.1.400.ga245620fadb-goog

