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SubjectRe: LTP hugemmap05 test case failure on arm64 with linux-next (next-20190613)
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So the problem is that ipcget_public() has held the semaphore "ids->rwsem" for
too long seems unnecessarily and then goes to sleep sometimes due to direct
reclaim (other times LTP hugemmap05 [1] has hugetlb_file_setup() returns
-ENOMEM),

[  788.765739][ T1315] INFO: task hugemmap05:5001 can't die for more than 122
seconds.
[  788.773512][ T1315] hugemmap05      R  running task    25600  5001      1
0x0000000d
[  788.781348][ T1315] Call trace:
[  788.784536][ T1315]  __switch_to+0x2e0/0x37c
[  788.788848][ T1315]  try_to_free_pages+0x614/0x934
[  788.793679][ T1315]  __alloc_pages_nodemask+0xe88/0x1d60
[  788.799030][ T1315]  alloc_fresh_huge_page+0x16c/0x588
[  788.804206][ T1315]  alloc_surplus_huge_page+0x9c/0x278
[  788.809468][ T1315]  hugetlb_acct_memory+0x114/0x5c4
[  788.814469][ T1315]  hugetlb_reserve_pages+0x170/0x2b0
[  788.819662][ T1315]  hugetlb_file_setup+0x26c/0x3a8
[  788.824600][ T1315]  newseg+0x220/0x63c
[  788.828490][ T1315]  ipcget+0x570/0x674
[  788.832377][ T1315]  ksys_shmget+0x90/0xc4
[  788.836525][ T1315]  __arm64_sys_shmget+0x54/0x88
[  788.841282][ T1315]  el0_svc_handler+0x19c/0x26c
[  788.845952][ T1315]  el0_svc+0x8/0xc

and then all other processes are waiting on the semaphore causes lock
contentions,

[  788.849583][ T1315] INFO: task hugemmap05:5027 blocked for more than 122
seconds.
[  788.857119][ T1315]       Tainted: G        W         5.2.0-rc6-next-20190624
#2
[  788.864566][ T1315] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs"
disables this message.
[  788.873139][ T1315] hugemmap05      D26960  5027   5026 0x00000000
[  788.879395][ T1315] Call trace:
[  788.882576][ T1315]  __switch_to+0x2e0/0x37c
[  788.886901][ T1315]  __schedule+0xb74/0xf0c
[  788.891136][ T1315]  schedule+0x60/0x168
[  788.895097][ T1315]  rwsem_down_write_slowpath+0x5a0/0x8c8
[  788.900653][ T1315]  down_write+0xc0/0xc4
[  788.904715][ T1315]  ipcget+0x74/0x674
[  788.908516][ T1315]  ksys_shmget+0x90/0xc4
[  788.912664][ T1315]  __arm64_sys_shmget+0x54/0x88
[  788.917420][ T1315]  el0_svc_handler+0x19c/0x26c
[  788.922088][ T1315]  el0_svc+0x8/0xc

Ideally, it seems only ipc_findkey() and newseg() in this path needs to hold the
semaphore to protect concurrency access, so it could just be converted to a
spinlock instead.

[1] ./hugemmap05 -s -m

https://github.com/linux-test-project/ltp/blob/master/testcases/kernel/mem/huget
lb/hugemmap/hugemmap05.c

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