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From: Naoya Horiguchi To: Tong Tiangen Cc: Andrew Morton , Naoya Horiguchi , Miaohe Lin , wangkefeng.wang@huawei.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm: memory-failure: use rcu lock instead of tasklist_lock when collect_procs() Message-ID: <20230821041325.GA2341515@ik1-406-35019.vs.sakura.ne.jp> References: <20230821022534.1381092-1-tongtiangen@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20230821022534.1381092-1-tongtiangen@huawei.com> X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 7DE7E8000D X-Stat-Signature: s7i9rwphc8gyt7d1sf3jnfednnnw8wt3 X-HE-Tag: 1692591216-455641 X-HE-Meta: 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 Fm/k29GQ 2BeX1P5E+Z9PdkXN5IdfaJIKCch3cFXB4AbP2bZTCVswrz+pKoIKmGlQ3cQhsKu+bZ9v5s4SD+R/XtXBYtb9TQa8Vpyb7uo54FlvqiGyWrD/KpkNVpQWurWmHGqLICxTpskyn/6LVuRhn77kI/auw2G46QYy6BGzW77/g+HlFO5mWtUIV8YGPmF0Xbk7nitIcl4M/INEeHfvQazw= 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 Mon, Aug 21, 2023 at 10:25:34AM +0800, Tong Tiangen wrote: > We found a softlock issue in our test, analyzed the logs, and found that > the relevant CPU call trace as follows: > > CPU0: > _do_fork > -> copy_process() > -> write_lock_irq(&tasklist_lock) //Disable irq,waiting for > //tasklist_lock > > CPU1: > wp_page_copy() > ->pte_offset_map_lock() > -> spin_lock(&page->ptl); //Hold page->ptl > -> ptep_clear_flush() > -> flush_tlb_others() ... > -> smp_call_function_many() > -> arch_send_call_function_ipi_mask() > -> csd_lock_wait() //Waiting for other CPUs respond > //IPI > > CPU2: > collect_procs_anon() > -> read_lock(&tasklist_lock) //Hold tasklist_lock > ->for_each_process(tsk) > -> page_mapped_in_vma() > -> page_vma_mapped_walk() > -> map_pte() > ->spin_lock(&page->ptl) //Waiting for page->ptl > > We can see that CPU1 waiting for CPU0 respond IPI，CPU0 waiting for CPU2 > unlock tasklist_lock, CPU2 waiting for CPU1 unlock page->ptl. As a result, > softlockup is triggered. > > For collect_procs_anon(), we will not modify the tasklist, but only perform > read traversal. Therefore, we can use rcu lock instead of spin lock > tasklist_lock, from this, we can break the softlock chain above. > > The same logic can also be applied to: > - collect_procs_file() > - collect_procs_fsdax() > - collect_procs_ksm() > - find_early_kill_thread() > > Signed-off-by: Tong Tiangen Acked-by: Naoya Horiguchi > --- > Changes since RFC[1]: > - 1. According to Naoya's suggestion, modify the tasklist_lock in the > comment about locking order in mm/filemap.c. > - 2. According to Kefeng's suggestion, optimize the implementation of > find_early_kill_thread() without functional changes. > - 3. Modify the title description. > > [1] https://lore.kernel.org/lkml/20230815130154.1100779-1-tongtiangen@huawei.com/