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

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

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@canonical.com>
Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Cc: Christoph Lameter <cl@linux.com>

--
Hi Andrew,

We ran into this bug when working towards enabling memory corruption
on arm64. The patch was tested on an arm64 platform running v4.12-rc2
with the series to enable memory corruption handling[0].

Please consider merging as a fix for the 4.12 release.

Thanks,
Punit

[0] https://www.spinics.net/lists/arm-kernel/msg581657.html
---
 mm/migrate.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 89a0a1707f4c..187abd1526df 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1201,10 +1201,8 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
 	 * tables or check whether the hugepage is pmd-based or not before
 	 * kicking migration.
 	 */
-	if (!hugepage_migration_supported(page_hstate(hpage))) {
-		putback_active_hugepage(hpage);
+	if (!hugepage_migration_supported(page_hstate(hpage)))
 		return -ENOSYS;
-	}
 
 	new_hpage = get_new_page(hpage, private, &result);
 	if (!new_hpage)
-- 
2.11.0

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