On Tue 17-01-17 09:06:04, Huang, Ying wrote: > Michal Hocko <mhocko@xxxxxxxxxx> writes: > > > Hi, > > I am seeing a lot of preempt unsafe warnings with the current mmotm and > > I assume that this patchset has introduced the issue. I haven't checked > > more closely but get_swap_page didn't use this_cpu_ptr before "mm/swap: > > add cache for swap slots allocation" > > > > [ 57.812314] BUG: using smp_processor_id() in preemptible [00000000] code: kswapd0/527 > > [ 57.814360] caller is debug_smp_processor_id+0x17/0x19 > > [ 57.815237] CPU: 1 PID: 527 Comm: kswapd0 Tainted: G W 4.9.0-mmotm-00135-g4e9a9895ebef #1042 > > [ 57.816019] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.1-1 04/01/2014 > > [ 57.816019] ffffc900001939c0 ffffffff81329c60 0000000000000001 ffffffff81a0ce06 > > [ 57.816019] ffffc900001939f0 ffffffff81343c2a 00000000000137a0 ffffea0000dfd2a0 > > [ 57.816019] ffff88003c49a700 ffffc90000193b10 ffffc90000193a00 ffffffff81343c53 > > [ 57.816019] Call Trace: > > [ 57.816019] [<ffffffff81329c60>] dump_stack+0x68/0x92 > > [ 57.816019] [<ffffffff81343c2a>] check_preemption_disabled+0xce/0xe0 > > [ 57.816019] [<ffffffff81343c53>] debug_smp_processor_id+0x17/0x19 > > [ 57.816019] [<ffffffff8115f06f>] get_swap_page+0x19/0x183 > > [ 57.816019] [<ffffffff8114e01d>] shmem_writepage+0xce/0x38c > > [ 57.816019] [<ffffffff81148916>] shrink_page_list+0x81f/0xdbf > > [ 57.816019] [<ffffffff81149652>] shrink_inactive_list+0x2ab/0x594 > > [ 57.816019] [<ffffffff8114a22f>] shrink_node_memcg+0x4c7/0x673 > > [ 57.816019] [<ffffffff8114a49f>] shrink_node+0xc4/0x282 > > [ 57.816019] [<ffffffff8114a49f>] ? shrink_node+0xc4/0x282 > > [ 57.816019] [<ffffffff8114b8cb>] kswapd+0x656/0x834 > > [ 57.816019] [<ffffffff8114b275>] ? mem_cgroup_shrink_node+0x2e1/0x2e1 > > [ 57.816019] [<ffffffff81069fb4>] ? call_usermodehelper_exec_async+0x124/0x12d > > [ 57.816019] [<ffffffff81073621>] kthread+0xf9/0x101 > > [ 57.816019] [<ffffffff81660198>] ? _raw_spin_unlock_irq+0x2c/0x4a > > [ 57.816019] [<ffffffff81073528>] ? kthread_park+0x5a/0x5a > > [ 57.816019] [<ffffffff81069e90>] ? umh_complete+0x25/0x25 > > [ 57.816019] [<ffffffff81660b07>] ret_from_fork+0x27/0x40 > > Sorry for bothering, we should have tested this before. I am always running my tests with CONFIG_DEBUG_PREEMPT=y which is what has caught this one. [...] > > would be a way to go but the function takes a sleeping lock so disabling > > the preemption is not a way forward. So this is either preempt safe > > for some reason - which should be IMHO documented in a comment - and > > raw_cpu_ptr can be used or this needs a deeper thought. > > Thanks for pointing out this. > > We think this is preempt safe. During the development, we have > considered the possible preemption between getting the per-CPU pointer > and its usage, and implemented the code to make it work at that > situation. We will change the code to use raw_cpu_ptr() and add a > comment for it. FWIW s@this_cpu_ptr@raw_cpu_ptr@ which I am using as a workaround now hasn't seemed to cause any issue. At least nothing observable like a crash. Thanks! -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>