Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
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, hughd@google.com
Subject: [PATCH v3 8/9] mm: do not use atomic_set_release in page_ref_unfreeze()
Date:   Wed, 26 Jan 2022 18:34:28 +0000
Message-Id: <20220126183429.1840447-9-pasha.tatashin@soleen.com>
In-Reply-To: <20220126183429.1840447-1-pasha.tatashin@soleen.com>
References: <20220126183429.1840447-1-pasha.tatashin@soleen.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

In we set the old _refcount value after verifying that the old value was
indeed 0.

VM_BUG_ON_PAGE(page_count(page) != 0, page);
< the _refcount may change here>
atomic_set_release(&page->_refcount, count);

To avoid the smal gap where _refcount may change lets verify the time
of the _refcount at the time of the set operation.

Use atomic_xchg_release() and at the set time verify that the value
was 0.

Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
---
 include/linux/page_ref.h | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
index 243fc60ae6c8..9efabeff4e06 100644
--- a/include/linux/page_ref.h
+++ b/include/linux/page_ref.h
@@ -322,10 +322,9 @@ static inline int folio_ref_freeze(struct folio *folio, int count)
 
 static inline void page_ref_unfreeze(struct page *page, int count)
 {
-	VM_BUG_ON_PAGE(page_count(page) != 0, page);
-	VM_BUG_ON(count == 0);
+	int old_val = atomic_xchg_release(&page->_refcount, count);
 
-	atomic_set_release(&page->_refcount, count);
+	VM_BUG_ON_PAGE(count == 0 || old_val != 0, page);
 	if (page_ref_tracepoint_active(page_ref_unfreeze))
 		__page_ref_unfreeze(page, count);
 }
-- 
2.35.0.rc0.227.g00780c9af4-goog