Re: [PATCH] fs: proc: use down_read_killable in proc_pid_cmdline_read()

[Yang Shi <yang.shi@linux.alibaba.com>]

On 2/20/18 11:49 AM, 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.
>
> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
> Cc: Alexey Dobriyan <adobriyan@gmail.com>
> ---
>   fs/proc/base.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 9298324..44b6f8f 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -242,7 +242,9 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
>   		goto out_mmput;
>   	}
>   
> -	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;

Should goto out_free_page, amended patch as below.

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 9298324..057e6bd 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -242,7 +242,9 @@ static ssize_t proc_pid_cmdline_read(struct file 
*file, char __user *buf,
                 goto out_mmput;
         }

-       down_read(&mm->mmap_sem);
+       rv = down_read_killable(&mm->mmap_sem);
+       if (rv)
+               goto out_free_page;
         arg_start = mm->arg_start;
         arg_end = mm->arg_end;
         env_start = mm->env_start;