Andrew Morton [off-list ref] writes:

On Wed, 24 May 2017 16:47:28 +0100 Punit Agrawal [off-list ref] wrote:

quoted

On failing to migrate a page, soft_offline_huge_page() performs the
necessary update to the hugepage ref-count. When
!hugepage_migration_supported() , unmap_and_move_hugepage() also
decrements the page ref-count for the hugepage. The combined behaviour
leaves the ref-count in an inconsistent state.

This leads to soft lockups when running the overcommitted hugepage test
from mce-tests suite.

Soft offlining pfn 0x83ed600 at process virtual address 0x400000000000
soft offline: 0x83ed600: migration failed 1, type
1fffc00000008008 (uptodate|head)
INFO: rcu_preempt detected stalls on CPUs/tasks:
 Tasks blocked on level-0 rcu_node (CPUs 0-7): P2715
  (detected by 7, t=5254 jiffies, g=963, c=962, q=321)
  thugetlb_overco R  running task        0  2715   2685 0x00000008
  Call trace:
  [<ffff000008089f90>] dump_backtrace+0x0/0x268
  [<ffff00000808a2d4>] show_stack+0x24/0x30
  [<ffff000008100d34>] sched_show_task+0x134/0x180
  [<ffff0000081c90fc>] rcu_print_detail_task_stall_rnp+0x54/0x7c
  [<ffff00000813cfd4>] rcu_check_callbacks+0xa74/0xb08
  [<ffff000008143a3c>] update_process_times+0x34/0x60
  [<ffff0000081550e8>] tick_sched_handle.isra.7+0x38/0x70
  [<ffff00000815516c>] tick_sched_timer+0x4c/0x98
  [<ffff0000081442e0>] __hrtimer_run_queues+0xc0/0x300
  [<ffff000008144fa4>] hrtimer_interrupt+0xac/0x228
  [<ffff0000089a56d4>] arch_timer_handler_phys+0x3c/0x50
  [<ffff00000812f1bc>] handle_percpu_devid_irq+0x8c/0x290
  [<ffff0000081297fc>] generic_handle_irq+0x34/0x50
  [<ffff000008129f00>] __handle_domain_irq+0x68/0xc0
  [<ffff0000080816b4>] gic_handle_irq+0x5c/0xb0

Fix this by dropping the ref-count decrement in
unmap_and_move_hugepage() when !hugepage_migration_supported().

Fixes: 32665f2bbfed ("mm/migrate: correct failure handling if !hugepage_migration_support()")
Reported-by: Manoj Iyer <redacted>
Signed-off-by: Punit Agrawal <redacted>
Cc: Joonsoo Kim <redacted>
Cc: Naoya Horiguchi <redacted>
Cc: Wanpeng Li <redacted>
Cc: Christoph Lameter <redacted>

32665f2bbfed was three years ago.  Do you have any theory as to why
this took so long to be detected?

This only triggers on systems that enable memory failure handling
(ARCH_SUPPORTS_MEMORY_FAILURE) but not hugepage migration
(!ARCH_ENABLE_HUGEPAGE_MIGRATION).

I imagine this wasn't triggered as there aren't many systems running
this configuration.

And do you believe a -stable backport is warranted?

I'll defer to Horiguchi-san's judgement here.

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