Re: + mm-migrate-fix-ref-count-handling-when-hugepage_migration_supported-v2.patch added to -mm tree

[Punit Agrawal <punit.agrawal@arm.com>]

Mel Gorman <mgorman@techsingularity.net> writes:

> On Thu, May 25, 2017 at 03:46:23PM -0700, akpm@linux-foundation.org wrote:
>> ------------------------------------------------------
>> From: Punit Agrawal <punit.agrawal@arm.com>
>> Subject: mm/migrate: fix refcount handling when !hugepage_migration_supported()
>> 
>> 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
>> 
>> Address this by changing the putback_active_hugepage() in
>> soft_offline_huge_page() to putback_movable_pages().
>> 
>
> I've never looked too closely at how hardware poisoning and hugetlb pages
> migration is handled so I could easily have missed something but this
> changelog and patch confuses me.
>
> Surely if the inconsistency is between hugepage_migration_supported and
> !hugepage_migration_supported then the check in soft_offline_huge_page()
> should also be related to hugepage_migration_supported either in
> soft_offline_huge_page() or in putback_movable_pages()?

The first version of the patch did indeed make a change that was around
!hugepage_migration_supported() [0] which was effectively a revert of
32665f2bbfed ("mm/migrate: correct failure handling if
!hugepage_migration_support()").

But Horiguchi-san suggested that dropping the putback_active_hugepage()
from unmap_and_move_hugepage() will bring back the issue that
32665f2bbfed addressed it was safer to take the current approach. It
also matches the pattern followed for !hugepage.

I did update the changelog but perhaps not enough - would updating the
changelog to reflect this help make it clearer?

Thanks,
Punit

[0] https://www.spinics.net/lists/kernel/msg2516334.html