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[85.71.35.39]) by smtp.gmail.com with ESMTPSA id q20sm1498560wmc.14.2021.01.31.09.22.32 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 31 Jan 2021 09:22:32 -0800 (PST) Subject: Re: Very slow unlockall() From: Milan Broz To: linux-mm@kvack.org Cc: Linux Kernel Mailing List , Mikulas Patocka , Michal Hocko References: <70885d37-62b7-748b-29df-9e94f3291736@gmail.com> <20210108134140.GA9883@dhcp22.suse.cz> Message-ID: <36e72f8c-3e23-ec48-d8c5-402dc8cfb9c9@gmail.com> Date: Sun, 31 Jan 2021 18:22:31 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08/01/2021 15:39, Milan Broz wrote: > On 08/01/2021 14:41, Michal Hocko wrote: >> On Wed 06-01-21 16:20:15, Milan Broz wrote: >>> Hi, >>> >>> we use mlockall(MCL_CURRENT | MCL_FUTURE) / munlockall() in cryptsetup code >>> and someone tried to use it with hardened memory allocator library. >>> >>> Execution time was increased to extreme (minutes) and as we found, the problem >>> is in munlockall(). >>> >>> Here is a plain reproducer for the core without any external code - it takes >>> unlocking on Fedora rawhide kernel more than 30 seconds! >>> I can reproduce it on 5.10 kernels and Linus' git. >>> >>> The reproducer below tries to mmap large amount memory with PROT_NONE (later never used). >>> The real code of course does something more useful but the problem is the same. >>> >>> #include >>> #include >>> #include >>> #include >>> >>> int main (int argc, char *argv[]) >>> { >>> void *p = mmap(NULL, 1UL << 41, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); >>> >>> if (p == MAP_FAILED) return 1; >>> >>> if (mlockall(MCL_CURRENT | MCL_FUTURE)) return 1; >>> printf("locked\n"); >>> >>> if (munlockall()) return 1; >>> printf("unlocked\n"); >>> >>> return 0; >>> } >>> >>> In traceback I see that time is spent in munlock_vma_pages_range. >>> >>> [ 2962.006813] Call Trace: >>> [ 2962.006814] ? munlock_vma_pages_range+0xe7/0x4b0 >>> [ 2962.006814] ? vma_merge+0xf3/0x3c0 >>> [ 2962.006815] ? mlock_fixup+0x111/0x190 >>> [ 2962.006815] ? apply_mlockall_flags+0xa7/0x110 >>> [ 2962.006816] ? __do_sys_munlockall+0x2e/0x60 >>> [ 2962.006816] ? do_syscall_64+0x33/0x40 >>> ... >>> >>> Or with perf, I see >>> >>> # Overhead Command Shared Object Symbol >>> # ........ ....... ................. ..................................... >>> # >>> 48.18% lock [kernel.kallsyms] [k] lock_is_held_type >>> 11.67% lock [kernel.kallsyms] [k] ___might_sleep >>> 10.65% lock [kernel.kallsyms] [k] follow_page_mask >>> 9.17% lock [kernel.kallsyms] [k] debug_lockdep_rcu_enabled >>> 6.73% lock [kernel.kallsyms] [k] munlock_vma_pages_range >>> ... >>> >>> >>> Could please anyone check what's wrong here with the memory locking code? >>> Running it on my notebook I can effectively DoS the system :) >>> >>> Original report is https://gitlab.com/cryptsetup/cryptsetup/-/issues/617 >>> but this is apparently a kernel issue, just amplified by usage of munlockall(). >> >> Which kernel version do you see this with? Have older releases worked >> better? > > Hi, > > I tried 5.10 stable and randomly few kernels I have built on testing VM (5.3 was the oldest), > it seems to be very similar run time, so the problem is apparently old...(I can test some specific kernel version if it make any sense). > > For mainline (reproducer above): > > With 5.11.0-0.rc2.20210106git36bbbd0e234d.117.fc34.x86_64 (latest Fedora rawhide kernel build - many debug options are on) > > # time ./lock > locked > unlocked > > real 0m32.287s > user 0m0.001s > sys 0m32.126s > > > Today's Linus git - 5.11.0-rc2+ in my testing x86_64 VM (no extensive kernel debug options): > > # time ./lock > locked > unlocked > > real 0m4.172s > user 0m0.000s > sys 0m4.172s > > m. Hi, so because there is no response, is this expected behavior of memory management subsystem then? Thanks, Milan