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[34.85.216.189]) by smtp.gmail.com with ESMTPSA id 143sm1898710qkg.87.2021.12.08.12.35.59 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 08 Dec 2021 12:35:59 -0800 (PST) From: Pasha Tatashin 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 09/10] mm: do not use atomic_set_release in page_ref_unfreeze() Date: Wed, 8 Dec 2021 20:35:43 +0000 Message-Id: <20211208203544.2297121-10-pasha.tatashin@soleen.com> X-Mailer: git-send-email 2.34.1.400.ga245620fadb-goog In-Reply-To: <20211208203544.2297121-1-pasha.tatashin@soleen.com> References: <20211208203544.2297121-1-pasha.tatashin@soleen.com> MIME-Version: 1.0 X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 9478F10000A9 X-Stat-Signature: unh8ci9gbegxqwz876wcgr5fzdht7ene Authentication-Results: imf12.hostedemail.com; dkim=pass header.d=soleen.com header.s=google header.b="CZ/IogjH"; spf=pass (imf12.hostedemail.com: domain of pasha.tatashin@soleen.com designates 209.85.219.41 as permitted sender) smtp.mailfrom=pasha.tatashin@soleen.com; dmarc=none X-HE-Tag: 1638995761-649460 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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 --- 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 8c76bf3bf7e1..26676d3bcd58 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); }