[PATCH] mm/migrate: Fix ref-count handling when !hugepage_migration_supported()

[Naoya Horiguchi]

On Wed, May 24, 2017 at 12:56:10PM -0700, Andrew Morton wrote:
> On Wed, 24 May 2017 16:47:28 +0100 Punit Agrawal <punit.agrawal at arm.com> wrote:
> 
> > 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 <manoj.iyer at canonical.com>
> > Signed-off-by: Punit Agrawal <punit.agrawal at arm.com>
> > Cc: Joonsoo Kim <iamjoonsoo.kim at lge.com>
> > Cc: Naoya Horiguchi <n-horiguchi at ah.jp.nec.com>
> > Cc: Wanpeng Li <liwanp at linux.vnet.ibm.com>
> > Cc: Christoph Lameter <cl at linux.com>
> 
> 32665f2bbfed was three years ago.  Do you have any theory as to why
> this took so long to be detected?

My per-release testing only ran for "hugepage_migration_supported() == true"
setting (i.e. x86 with CONFIG_HUGETLB_PAGE=y). I need extend the coverage.
And other arch's developers recently have come to have interest in hugepage
migration.

>  And do you believe a -stable
> backport is warranted?

I agree to send the fix to stable, so the stable tag is wanted.

Cc: stable at kernel.org   # v3.14+

Thanks,
Naoya Horiguchi