On 3/21/18 9:50 AM, Yang Shi wrote:

On 3/21/18 6:14 AM, Michal Hocko wrote:

quoted

On Wed 21-03-18 05:31:19, Yang Shi wrote:

quoted

When running some mmap/munmap scalability tests with large memory (i.e.

quoted

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