Possible deadlock in v4.14.15 contention on shrinker_rwsem in shrink_slab()

From: Eric Wheeler <linux-mm@lists.ewheeler.net>
To: linux-mm@kvack.org
Subject: Possible deadlock in v4.14.15 contention on shrinker_rwsem in shrink_slab()
Date: Wed, 24 Jan 2018 23:57:42 +0000 (UTC)	[thread overview]
Message-ID: <alpine.LRH.2.11.1801242349220.30642@mail.ewheeler.net> (raw)

Hello all,

We are getting processes stuck with /proc/pid/stack listing the following:

[<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

I seem to be able to reproduce this somewhat reliably, it will likely be 
stuck by tomorrow morning. Since it does seem to take a day to hang, I was 
hoping to avoid a bisect and see if anyone has seen this behavior or knows 
it to be fixed in 4.15-rc.

Note that we are using zram as our only swap device, but at the time that 
it shrink_slab() failed to return, there was plenty of memory available 
and no swap was in use.

The machine is generally responsive, but `sync` will hang forever and our 
only way out is `echo b > /proc/sysrq-trigger`.

Please suggest any additional information you might need for testing, and 
I am happy to try patches.

Thank you for your help!

--
Eric Wheeler

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