Re: [PATCH 1/2] mm,vmscan: Kill global shrinker lock.

[Michal Hocko]

On Thu 25-01-18 11:04:23, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > On Wed 15-11-17 09:00:20, Johannes Weiner wrote:
> > > In any case, Minchan's lock breaking seems way preferable over that
> > > level of headscratching complexity for an unusual case like Shakeel's.
> > 
> > agreed! I would go the more complex way only if it turns out that early
> > break out causes some real problems.
> > 
> 
> Eric Wheeler wrote (at http://lkml.kernel.org/r/alpine.LRH.2.11.1801242349220.30642@xxxxxxxxxxxxxxxxx ):
> > Hello all,
> > 
> > We are getting processes stuck with /proc/pid/stack listing the following:
> 
> Yes, I think that this is a silent OOM lockup.
> 
> > 
> > [<ffffffffac0cd0d2>] io_schedule+0x12/0x40
> > [<ffffffffac1b4695>] __lock_page+0x105/0x150
> > [<ffffffffac1b4dc1>] pagecache_get_page+0x161/0x210
> > [<ffffffffac1d4ab4>] shmem_unused_huge_shrink+0x334/0x3f0
> > [<ffffffffac251546>] super_cache_scan+0x176/0x180
> > [<ffffffffac1cb6c5>] shrink_slab+0x275/0x460
> > [<ffffffffac1d0b8e>] shrink_node+0x10e/0x320
> > [<ffffffffac1d0f3d>] node_reclaim+0x19d/0x250
> > [<ffffffffac1be0aa>] get_page_from_freelist+0x16a/0xac0
> > [<ffffffffac1bed87>] __alloc_pages_nodemask+0x107/0x290
> > [<ffffffffac06dbc3>] pte_alloc_one+0x13/0x40
> > [<ffffffffac1ef329>] __pte_alloc+0x19/0x100
> > [<ffffffffac1f17b8>] alloc_set_pte+0x468/0x4c0
> > [<ffffffffac1f184a>] finish_fault+0x3a/0x70
> > [<ffffffffac1f369a>] __handle_mm_fault+0x94a/0x1190
> > [<ffffffffac1f3fa4>] handle_mm_fault+0xc4/0x1d0
> > [<ffffffffac0682a3>] __do_page_fault+0x253/0x4d0
> > [<ffffffffac068553>] do_page_fault+0x33/0x120
> > [<ffffffffac8019dc>] page_fault+0x4c/0x60
> > 
> > 
> > For some reason io_schedule is not coming back, so shrinker_rwsem never 
> > gets an up_read. When this happens, other processes like libvirt get stuck 
> > trying to start VMs with the /proc/pid/stack of libvirtd looking like so, 
> > while register_shrinker waits for shrinker_rwsem to be released:
> > 
> > [<ffffffffac7538d3>] call_rwsem_down_write_failed+0x13/0x20
> > [<ffffffffac1cb985>] register_shrinker+0x45/0xa0
> > [<ffffffffac250f68>] sget_userns+0x468/0x4a0
> > [<ffffffffac25106a>] mount_nodev+0x2a/0xa0
> > [<ffffffffac251be4>] mount_fs+0x34/0x150
> > [<ffffffffac2701f2>] vfs_kern_mount+0x62/0x120
> > [<ffffffffac272a0e>] do_mount+0x1ee/0xc50
> > [<ffffffffac27377e>] SyS_mount+0x7e/0xd0
> > [<ffffffffac003831>] do_syscall_64+0x61/0x1a0
> > [<ffffffffac80012c>] entry_SYSCALL64_slow_path+0x25/0x25
> > [<ffffffffffffffff>] 0xffffffffffffffff
> > 
> 
> If io_schedule() depends on somebody else's memory allocation request, that
> somebody else will call shrink_slab() and down_read_trylock(&shrinker_rwsem)
> will fail without making progress. This means that that somebody else will
> forever retry as long as should_continue_reclaim() returns true.

I would rather understand the problem than speculate here. I strongly
suspect somebody simply didn't unlock the page.
-- 
Michal Hocko
SUSE Labs