linux-kernel - Re: kcompactd hang during memory offlining

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Date:	Wed, 4 May 2016 00:16:42 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Reza Arbab <arbab@...ux.vnet.ibm.com>,
	Arnd Bergmann <arnd@...db.de>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Andrea Arcangeli <aarcange@...hat.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Mel Gorman <mgorman@...hsingularity.net>,
	David Rientjes <rientjes@...gle.com>,
	Michal Hocko <mhocko@...e.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Hugh Dickins <hughd@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: kcompactd hang during memory offlining

On 05/03/2016 07:02 PM, Reza Arbab wrote:
> Assume memory47 is the last online block left in node1. This will hang:
> 
> # echo offline > /sys/devices/system/node/node1/memory47/state
> 
> After a couple of minutes, the following pops up in dmesg:
> 
> INFO: task bash:957 blocked for more than 120 seconds.

Damn, can you test this patch? I hope it's just the simple mistake and kcompactd is
waiting for the kcompactd_max_order > 0 when it's woken up to actually exit.
No idea what happens if memory actually gets offlined during compaction's pfn scan...
but that wouldn't be new or specific to kcompactd...

----8<----
diff --git a/mm/compaction.c b/mm/compaction.c
index 481004c73c90..0e28981d4510 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1852,7 +1852,7 @@ void compaction_unregister_node(struct node *node)
 
 static inline bool kcompactd_work_requested(pg_data_t *pgdat)
 {
-       return pgdat->kcompactd_max_order > 0;
+       return pgdat->kcompactd_max_order > 0 || kthread_should_stop();
 }
 
 static bool kcompactd_node_suitable(pg_data_t *pgdat)
@@ -1916,6 +1916,8 @@ static void kcompactd_do_work(pg_data_t *pgdat)
                INIT_LIST_HEAD(&cc.freepages);
                INIT_LIST_HEAD(&cc.migratepages);
 
+               if (kthread_should_stop())
+                       return;
                status = compact_zone(zone, &cc);
 
                if (zone_watermark_ok(zone, cc.order, low_wmark_pages(zone),