Re: [linux-next: Tree for Jun 1] __khugepaged_exit rwsem_down_write_failed lockup

On Fri 03-06-16 11:55:49, Michal Hocko wrote:

On Fri 03-06-16 17:43:47, Sergey Senozhatsky wrote:

quoted

On (06/03/16 09:25), Michal Hocko wrote:

quoted

quoted

it's quite hard to trigger the bug (somehow), so I can't
follow up with more information as of now.

either I did something very silly fixing up the patch, or the
patch may be causing general protection faults on my system.

RIP collect_mm_slot() + 0x42/0x84
	khugepaged

So is this really collect_mm_slot called directly from khugepaged or is
some inlining going on there?

quoted

	prepare_to_wait_event
	maybe_pmd_mkwrite
	kthread
	_raw_sin_unlock_irq
	ret_from_fork
	kthread_create_on_node

collect_mm_slot() + 0x42/0x84 is

I guess that the problem is that I have missed that __khugepaged_exit
doesn't clear the cached khugepaged_scan.mm_slot. Does the following on
top fixes that?

That wouldn't be sufficient after a closer look. We need to do the same
from khugepaged_scan_mm_slot when atomic_inc_not_zero fails. So I guess
it would be better to stick it into collect_mm_slot.

Thanks for your testing!
---

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 6574c62ca4a3..0432581fb87c 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c

@@ -2011,6 +2011,9 @@ static void collect_mm_slot(struct mm_slot *mm_slot)
 		/* khugepaged_mm_lock actually not necessary for the below */
 		free_mm_slot(mm_slot);
 		mmdrop(mm);
+
+		if (khugepaged_scan.mm_slot == mm_slot)
+			khugepaged_scan.mm_slot = NULL;
 	}
 }
 

-- 
Michal Hocko
SUSE Labs

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