lkml.org 
[lkml]   [2018]   [Feb]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH] fs: proc: use down_read_killable in proc_pid_cmdline_read()
From
Date


On 2/21/18 11:57 AM, Alexey Dobriyan wrote:
> On Tue, Feb 20, 2018 at 03:38:24PM -0800, Yang Shi wrote:
>>
>> On 2/20/18 2:38 PM, Alexey Dobriyan wrote:
>>> On Wed, Feb 21, 2018 at 03:49:29AM +0800, Yang Shi wrote:
>>>> When running vm-scalability with large memory (> 300GB), the below hung
>>>> task issue happens 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
>>>> [<ffffffff81253c95>] ? do_filp_open+0xa5/0x100
>>>> [<ffffffff81241d87>] __vfs_read+0x37/0x150
>>>> [<ffffffff812f824b>] ? security_file_permission+0x9b/0xc0
>>>> [<ffffffff81242266>] vfs_read+0x96/0x130
>>>> [<ffffffff812437b5>] SyS_read+0x55/0xc0
>>>> [<ffffffff8171a6da>] entry_SYSCALL_64_fastpath+0x1a/0xc5
>>>>
>>>> When manipulating a large mapping, the process may hold the mmap_sem for
>>>> long time, so reading /proc/<pid>/cmdline may be blocked in
>>>> uninterruptible state for long time.
>>>>
>>>> down_read_trylock() sounds too aggressive, and we already have killable
>>>> version APIs for semaphore, here use down_read_killable() to improve the
>>>> responsiveness.
>>>> - down_read(&mm->mmap_sem);
>>>> + rv = down_read_killable(&mm->mmap_sem);
>>>> + if (rv)
>>>> + goto out_mmput;
>>>> arg_start = mm->arg_start;
>>>> arg_end = mm->arg_end;
>>>> env_start = mm->env_start;
>>> Fix is incomplete
>> Yes, it is. Since I just ran into the above splat, so I just did the
>> minimum change.
>>
>>> 1) /proc/*/cmdline only wants to read 4 values atomically,
>>> those 4 values are basically random values and aren't
>>> related to VM part at all (after C/R went in, they are
>>> settable to arbitrary values)
>> Sorry, I don't get your point here. Could you please elaborate?
> I hoped there is some random spinlock those 4 values could be moved to
> but no.

Those 4 values might be changed by prctl_set_mm concurrently if
mm->mmap_sem is not acquired.

>
>>> 2) access_remote_vm() et al will do the same ->mmap_sem, and
>> Yes, it does. But, __access_remote_vm() is called by access_process_vm()
>> too, which is used by much more places, i.e. ptrace, so I was not sure
>> if it is preferred to convert to killable version. So, I leave it untouched.
> Yeah, but ->mmap_sem is taken 3 times per /proc/*/cmdline read
> and your scalability tests should trigger next backtrace right away.

Yes, however, I didn't run into it if mmap_sem is acquired earlier.

How about defining a killable version, like
__access_remote_vm_killable() which use down_read_killable(), then the
killable version can be used by proc/*/cmdline? There might be other
users in the future.

Thanks,
Yang

>
>>> 3) /proc/*/environ and get_cmdline() do the same.
>> They look suitable to use killable version.
>>
>> BTW, I just realized the code should go to out_free_page instead of
>> out_mmput. Will rectify in newer version once we decide the extra places
>> to use killable version.

\
 
 \ /
  Last update: 2018-02-22 00:13    [W:0.060 / U:1.032 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site