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
