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

[Yang Shi]

On 3/26/18 10:49 AM, Tetsuo Handa wrote:
> 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.

However, a global lock might be hard to know what it is used for, and 
might be abused again.

And, it may introduce unexpected contention for parallel reading for /proc

> 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?

Yes, definitely, this patch is aimed to remove the abuse to mmap_sem. 
The mmap_sem contention will be addressed separately.

Yang

> > 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(-)