Re: [RFC PATCH 1/8] mm: mmap: unmap large mapping by section

[Yang Shi]

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.

Yang

> 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