On Tue 17-04-18 23:39:19, Cyrill Gorcunov wrote: > On Tue, Apr 17, 2018 at 11:29:57AM -0700, 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) > > > > 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? > > Nope, we still can't. Look, the down_read part order the call with > sys_brk. while arg_lock orders prctl call itself. That said if > someone is calling sys_brk while we're in a middle of prctl it > should wait until prctl finished. But two simultaneous prcl > may proceed without taking a write lock using arg_lock as > a barrier. A small comment would be due. The changelog mentions this but it would be nicer to comment why we care about mmap_sem for read in prctl. -- Michal Hocko SUSE Labs
