Messages in this thread | | | Subject | Re: [RFC PATCH 1/8] mm: mmap: unmap large mapping by section | From | Yang Shi <> | Date | Wed, 21 Mar 2018 09:50:31 -0700 |
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On 3/21/18 6:14 AM, Michal Hocko wrote: > On Wed 21-03-18 05:31:19, Yang Shi wrote: >> When running some mmap/munmap scalability tests with large memory (i.e. >>> 300GB), the below hung task issue may happen occasionally. >> INFO: task ps:14018 blocked for more than 120 seconds. >> Tainted: G E 4.9.79-009.ali3000.alios7.x86_64 #1 >> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this >> message. >> ps D 0 14018 1 0x00000004 >> ffff885582f84000 ffff885e8682f000 ffff880972943000 ffff885ebf499bc0 >> ffff8828ee120000 ffffc900349bfca8 ffffffff817154d0 0000000000000040 >> 00ffffff812f872a ffff885ebf499bc0 024000d000948300 ffff880972943000 >> Call Trace: >> [<ffffffff817154d0>] ? __schedule+0x250/0x730 >> [<ffffffff817159e6>] schedule+0x36/0x80 >> [<ffffffff81718560>] rwsem_down_read_failed+0xf0/0x150 >> [<ffffffff81390a28>] call_rwsem_down_read_failed+0x18/0x30 >> [<ffffffff81717db0>] down_read+0x20/0x40 >> [<ffffffff812b9439>] proc_pid_cmdline_read+0xd9/0x4e0 > Slightly off-topic: > Btw. this sucks as well. Do we really need to take mmap_sem here? Do any > of > arg_start = mm->arg_start; > arg_end = mm->arg_end; > env_start = mm->env_start; > env_end = mm->env_end; > > change after exec or while the pid is already visible in proc? If yes > maybe we can use a dedicated lock.
Actually, Alexey Dobriyan had the same comment when he reviewed my very first patch (which changes down_read to down_read_killable at that place).
Those 4 values might be changed by prctl_set_mm() and prctl_set_mm_map() concurrently. They used to use down_read() to protect the change, but it looks not good enough to protect concurrent writing. So, Mateusz Guzik's commit ddf1d398e517e660207e2c807f76a90df543a217 ("prctl: take mmap sem for writing to protect against others") change it to down_write().
It seems mmap_sem can be replaced to a dedicated lock. How about defining a rwlock in mm_struct to protect those data? I will come up with a RFC patch for this.
However, this dedicated lock just can work around this specific case. I believe solving mmap_sem scalability issue aimed by the patch series is still our consensus.
Thanks, Yang
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