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(-)
