Re: [PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct

[Tetsuo Handa]

On 2018/03/24 9:36, Yang Shi wrote:
> And, the mmap_sem contention may cause unexpected issue like below:
> 
> 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

Yes, but

> 
> Both Alexey Dobriyan and Michal Hocko suggested to use dedicated lock
> for them to mitigate the abuse of mmap_sem.
> 
> So, introduce a new rwlock in mm_struct to protect the concurrent access
> to arg_start|end and env_start|end.

does arg_lock really help?

I wonder whether per "struct mm_struct" granularity is needed if arg_lock
protects only a few atomic reads. A global lock would be sufficient.

Also, even if we succeeded to avoid mmap_sem contention at that location,
won't we after all get mmap_sem contention messages a bit later, for
access_remote_vm() holds mmap_sem which would lead to traces like above
if mmap_sem is already contended?

> 
> Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> ---
>  fs/proc/base.c           | 8 ++++----
>  include/linux/mm_types.h | 2 ++
>  kernel/fork.c            | 1 +
>  kernel/sys.c             | 6 ++++++
>  mm/init-mm.c             | 1 +
>  5 files changed, 14 insertions(+), 4 deletions(-)