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[194.45.78.10]) by smtp.gmail.com with ESMTPSA id ku15-20020a170907788f00b009571293d6acsm8373416ejc.59.2023.04.27.03.47.29 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 27 Apr 2023 03:47:30 -0700 (PDT) From: Jesper Dangaard Brouer X-Google-Original-From: Jesper Dangaard Brouer Message-ID: Date: Thu, 27 Apr 2023 12:47:28 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.9.1 Cc: brouer@redhat.com, lorenzo@kernel.org, =?UTF-8?Q?Toke_H=c3=b8iland-J=c3=b8rgensen?= , bpf@vger.kernel.org, "David S. Miller" , Jakub Kicinski , Paolo Abeni , Andrew Morton , willy@infradead.org Subject: Re: [PATCH RFC net-next/mm V1 1/3] page_pool: Remove workqueue in new shutdown scheme To: Yunsheng Lin , Ilias Apalodimas , netdev@vger.kernel.org, Eric Dumazet , linux-mm@kvack.org, Mel Gorman References: <168244288038.1741095.1092368365531131826.stgit@firesoul> <168244293875.1741095.10502498932946558516.stgit@firesoul> <48661b51-1cbb-e3e0-a909-6d0a1532733a@huawei.com> In-Reply-To: <48661b51-1cbb-e3e0-a909-6d0a1532733a@huawei.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Queue-Id: 1C0B74001E X-Rspamd-Server: rspam09 X-Stat-Signature: 9hma5abk6j1hiy5tez8kjuifoi1661fp X-HE-Tag: 1682592453-340919 X-HE-Meta: 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 M62IRo0R XnjddKgB6oammp7T626CX0T7mZ+S8twOzjYe3jbWFPZGYY6ggcClFWBuXlUmfKBMHrRSJNV7f3NvzIhX2d16D2P09xid3L829S9nvUveUFP+jcQO+e4NCNZ+jg1qRzlXVZU1TLMSIHaoP3fLunFsOHTnXFSmJJFZ0eoz9BeAxRShwzFy0zyw1JGx5uXEToH5amcFbXXHOA/ky5qT7qEo15lnyQi56YjGegVZBct4Vl+9MLkDw/0pcztY6wEBW3oPNJD8QQiyZNp+kSNrOKUiqgW0QHV2/P+4Yx8j8ndww56YO0pkEmmJ6kQi7C+oB7zJiseph0r+uGdb21vzao2nOvJAXnBNR7OZ0BhQe6x3+jQDiD6f9kXoS6ucXgK8Xj6JJvtC0g/KGizRBv+4On9i/Pl2eFtpxilPMHro8jvBpyHGSuJ7DcaLbsWwB7/5vrgnUC66qhOuivGjO4NXo+KzetnjizpHZiwSU+A1VjMaRp1UqMJM= 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 27/04/2023 02.57, Yunsheng Lin wrote: > On 2023/4/26 1:15, Jesper Dangaard Brouer wrote: >> @@ -609,6 +609,8 @@ void page_pool_put_defragged_page(struct page_pool *pool, struct page *page, >> recycle_stat_inc(pool, ring_full); >> page_pool_return_page(pool, page); >> } >> + if (pool->p.flags & PP_FLAG_SHUTDOWN) >> + page_pool_shutdown_attempt(pool); > > It seems we have allowed page_pool_shutdown_attempt() to be called > concurrently here, isn't there a time window between atomic_inc_return_relaxed() > and page_pool_inflight() for pool->pages_state_release_cnt, which may cause > double calling of page_pool_free()? > Yes, I think that is correct. I actually woke up this morning thinking of this case of double freeing, and this time window. Thanks for spotting and confirming this issue. Basically: Two concurrent CPUs executing page_pool_shutdown_attempt() can both end-up seeing inflight equal zero, resulting in both of them kfreeing the memory (in page_pool_free()) as they both think they are the last user of PP instance. I've been thinking how to address this. This is my current idea: (1) Atomic variable inc and test (or cmpxchg) that resolves last user race. (2) Defer free to call_rcu callback to let other CPUs finish. (3) Might need rcu_read_lock() in page_pool_shutdown_attempt(). --Jesper