On 4/17/18 11:29 AM, Andrew Morton wrote:
On Sun, 15 Apr 2018 02:24:51 +0800 Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> wrote:mmap_sem is on the hot path of kernel, and it very contended, but it is abused too. It is used to protect arg_start|end and evn_start|end when reading /proc/$PID/cmdline and /proc/$PID/environ, but it doesn't make sense since those proc files just expect to read 4 values atomically and not related to VM, they could be set to arbitrary values by C/R. 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 Both Alexey Dobriyan and Michal Hocko suggested to use dedicated lock for them to mitigate the abuse of mmap_sem. So, introduce a new spinlock in mm_struct to protect the concurrent access to arg_start|end, env_start|end and others, as well as replace write map_sem to read to protect the race condition between prctl and sys_brk which might break check_data_rlimit(), and makes prctl more friendly to other VM operations.(We should move check_data_rlimit() out of the .h file)
I don't get the point, check_data_rlimit() is still used by both prctl and sys_brk.
It seems inconsistent to be using mmap_sem to protect ->start_brk and friends in sys_brk(). We've already declared that these are protected by arg_lock so that's what we should be using? And getting this consistent should permit us to stop using mmap_sem in prctl() altogether?
Cyrill already helped to elaborate the reason. arg_lock just can protect the concurrent access of brk between prctl calls, but not sys_brk.
Thanks, Yang
