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Crashes/hung tasks with z3pool under memory pressure

Hi all,

we are observing crashes with z3pool under memory pressure. The kernel version
used to reproduce the problem is v4.16-11827-g5d1365940a68, but the problem was
also seen with v4.14 based kernels.

For simplicity, here is a set of shortened logs. A more complete log is
available at [1].

------------[ cut here ]------------
WARNING: CPU: 2 PID: 594 at kernel/sched/core.c:3212 preempt_count_add+0x90/0xa0
Modules linked in:
CPU: 2 PID: 594 Comm: memory-eater Not tainted 4.16.0-yocto-standard+ #8
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.10.2-1 04/01/2014
RIP: 0010:preempt_count_add+0x90/0xa0
RSP: 0000:ffffb12740db7750 EFLAGS: 00010286
RAX: 0000000000000000 RBX: 0000000000000001 RCX: 00000000000000f6
RDX: 00000000000000f6 RSI: 0000000000000082 RDI: 00000000ffffffff
RBP: fffff00480f357a0 R08: 000000000000004a R09: 00000000000001ad
R10: ffffb12740db77e0 R11: 0000000000000000 R12: ffff9cbc7e265d10
R13: ffff9cbc7cd5e000 R14: ffff9cbc7a7000d8 R15: fffff00480f35780
FS:  00007f5140791700(0000) GS:ffff9cbc7fd00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f513260f000 CR3: 0000000032086000 CR4: 00000000000006e0
Call Trace:
WARNING: CPU: 1 PID: 92 at mm/z3fold.c:278 release_z3fold_page_locked+0x25/0x40
Modules linked in:
INFO: rcu_preempt self-detected stall on CPU
	2-...!: (20958 ticks this GP) idle=5da/1/4611686018427387906
	softirq=4104/4113 fqs=11 
RIP: 0010:queued_spin_lock_slowpath+0x132/0x190
RSP: 0000:ffffb12740db7750 EFLAGS: 00000202 ORIG_RAX: ffffffffffffff13
RAX: 0000000000100101 RBX: ffff9cbc7a7000c8 RCX: 0000000000000001
RDX: 0000000000000101 RSI: 0000000000000001 RDI: 0000000000000101
RBP: 0000000000000000 R08: ffff9cbc7fc21240 R09: ffffffffaf19c900
R10: ffffb12740db75a0 R11: 0000000000000010 R12: fffff00480522d20
R13: ffff9cbc548b4000 R14: ffff9cbc7a7000d8 R15: fffff00480522d00
 ? __zswap_pool_current+0x80/0x90

With lock debugging enabled, the log is a bit different, but similar.

turning off the locking correctness validator.
depth: 48  max: 48!
48 locks held by memory-eater/619:
 #0: 000000002da807ce (&mm->mmap_sem){++++}, at: __do_page_fault+0x122/0x5a0
 #1: 0000000012fa6629 (&(&pool->lock)->rlock#3){+.+.}, at: z3fold_zpool_shrink+0x47/0x3e0
 #2: 00000000c85f45dd (&(&zhdr->page_lock)->rlock){+.+.}, at: z3fold_zpool_shrink+0xb7/0x3e0
 #3: 00000000876f5fdc (&(&zhdr->page_lock)->rlock){+.+.}, at: z3fold_zpool_shrink+0xb7/0x3e0
watchdog: BUG: soft lockup - CPU#0 stuck for 22s! [memory-eater:613]
Modules linked in:
irq event stamp: 1435394
hardirqs last  enabled at (1435393): [<ffffffff9b90ed81>]
hardirqs last disabled at (1435394): [<ffffffff9b907baa>] __schedule+0xba/0xbb0
softirqs last  enabled at (1434508): [<ffffffff9bc0027c>] __do_softirq+0x27c/0x516
softirqs last disabled at (1434323): [<ffffffff9b069a29>] irq_exit+0xa9/0xc0
CPU: 0 PID: 613 Comm: memory-eater Tainted: G        W
4.16.0-yocto-standard+ #9
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.10.2-1 04/01/2014
RIP: 0010:queued_spin_lock_slowpath+0x177/0x1a0
RSP: 0000:ffffa61f80e074e0 EFLAGS: 00000246 ORIG_RAX: ffffffffffffff13
RAX: 0000000000000000 RBX: ffff9704379cba08 RCX: ffff97043fc22080
RDX: 0000000000000001 RSI: ffffffff9c05f6a0 RDI: 0000000000040000
RBP: 0000000000000000 R08: ffffffff9b1f2215 R09: 0000000000000000
R10: ffffa61f80e07490 R11: ffff9704379cba20 R12: ffff97043fc22080
R13: ffffde77404e0920 R14: ffff970413824000 R15: ffff9704379cba00
FS:  00007f8c6317f700(0000) GS:ffff97043fc00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f8c43f3d010 CR3: 000000003aba0000 CR4: 00000000000006f0
Call Trace:

The problem is easy to reproduce. Please see [2] and the other files
at [3] for details. Various additional crash logs, observed with
chromeos-4.14, are available at [4].

Please let me know if there is anything else I can do to help solving
or debugging the problem. I had a look into the code, but I must admit
that its locking is a mystery to me.


[1] http://server.roeck-us.net/qemu/z3pool/crashdump
[2] http://server.roeck-us.net/qemu/z3pool/README
[3] http://server.roeck-us.net/qemu/z3pool/
[4] https://bugs.chromium.org/p/chromium/issues/detail?id=822360