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 15:36:12 -0700 |
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On 3/21/18 2:23 PM, Michal Hocko wrote: > On Wed 21-03-18 10:16:41, Yang Shi wrote: >> >> On 3/21/18 9:50 AM, Yang Shi wrote: >>> >>> 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. >> BTW, this is not the only place to acquire mmap_sem in >> proc_pid_cmdline_read(), it calls access_remote_vm() which need acquire >> mmap_sem too, so the mmap_sem scalability issue will be hit sooner or later. > Ohh, absolutely. mmap_sem is unfortunatelly abused and it would be great > to remove that. munmap should perform much better. How to do that safely
Yes, agree. We are on the same page.
> is a different question. I am not yet convinced that tearing down a vma > in batches is safe. The vast majority of time is spent on tearing down
You can try my patches. I did full LTP test and running multiple kernel build in parallel. It survives.
> pages and that is quite easy to move out of the write lock. That would > be an improvement already and it should be risk safe. If even that is > not sufficient then using range locking should help a lot. There > shouldn't be really any other address space operations within the range > most of the time so this would be basically non-contended access.
It might depend on how the range is defined. Too big range may lead to surprisingly more contention, but too small range may bring in too much lock/unlock operations.
Thanks, Yang
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