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[2003:cb:c708:1800:42bd:3cac:d22a:3c62]) by smtp.gmail.com with ESMTPSA id r4-20020a1c2b04000000b0038a0e15ee13sm2637525wmr.8.2022.03.15.08.45.13 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 15 Mar 2022 08:45:14 -0700 (PDT) Message-ID: <4cb789a5-c49c-f095-1f7e-67be65ba508a@redhat.com> Date: Tue, 15 Mar 2022 16:45:13 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.6.2 Subject: Re: [PATCH] mm/migrate: fix race between lock page and clear PG_Isolated To: Andrew Morton , Andrew Yang Cc: Matthias Brugger , Matthew Wilcox , Vlastimil Babka , David Howells , William Kucharski , Yang Shi , Marc Zyngier , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linux-mediatek@lists.infradead.org, wsd_upstream@mediatek.com, Nicholas Tang , Kuan-Ying Lee References: <20220315030515.20263-1-andrew.yang@mediatek.com> <20220314212127.a2797926ee0ef8a7ad05dcaa@linux-foundation.org> From: David Hildenbrand Organization: Red Hat In-Reply-To: <20220314212127.a2797926ee0ef8a7ad05dcaa@linux-foundation.org> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: B6EB91C000B X-Stat-Signature: pcq38131ipgzx3g9rn5k6kymkfktji7j Authentication-Results: imf20.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=bEimwjB7; dmarc=pass (policy=none) header.from=redhat.com; spf=none (imf20.hostedemail.com: domain of david@redhat.com has no SPF policy when checking 170.10.133.124) smtp.mailfrom=david@redhat.com X-Rspam-User: X-HE-Tag: 1647359117-636706 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: On 15.03.22 05:21, Andrew Morton wrote: > On Tue, 15 Mar 2022 11:05:15 +0800 Andrew Yang wrote: > >> When memory is tight, system may start to compact memory for large >> continuous memory demands. If one process tries to lock a memory page >> that is being locked and isolated for compaction, it may wait a long time >> or even forever. This is because compaction will perform non-atomic >> PG_Isolated clear while holding page lock, this may overwrite PG_waiters >> set by the process that can't obtain the page lock and add itself to the >> waiting queue to wait for the lock to be unlocked. >> >> CPU1 CPU2 >> lock_page(page); (successful) >> lock_page(); (failed) >> __ClearPageIsolated(page); SetPageWaiters(page) (may be overwritten) >> unlock_page(page); >> >> The solution is to not perform non-atomic operation on page flags while >> holding page lock. > > Sure, the non-atomic bitop optimization is really risky and I suspect > we reach for it too often. Or at least without really clearly > demonstrating that it is safe, and documenting our assumptions. I agree. IIRC, non-atomic variants are mostly only safe while the refcount is 0. Everything else is just absolutely fragile. -- Thanks, David / dhildenb