From 73444bc4d8f92e46a20cb6bd3342fc2ea75c6787 Mon Sep 17 00:00:00 2001 From: Mel Gorman Date: Tue, 8 Jan 2019 15:23:39 -0800 Subject: mm, page_alloc: do not wake kswapd with zone lock held syzbot reported the following regression in the latest merge window and it was confirmed by Qian Cai that a similar bug was visible from a different context. ====================================================== WARNING: possible circular locking dependency detected 4.20.0+ #297 Not tainted ------------------------------------------------------ syz-executor0/8529 is trying to acquire lock: 000000005e7fb829 (&pgdat->kswapd_wait){....}, at: __wake_up_common_lock+0x19e/0x330 kernel/sched/wait.c:120 but task is already holding lock: 000000009bb7bae0 (&(&zone->lock)->rlock){-.-.}, at: spin_lock include/linux/spinlock.h:329 [inline] 000000009bb7bae0 (&(&zone->lock)->rlock){-.-.}, at: rmqueue_bulk mm/page_alloc.c:2548 [inline] 000000009bb7bae0 (&(&zone->lock)->rlock){-.-.}, at: __rmqueue_pcplist mm/page_alloc.c:3021 [inline] 000000009bb7bae0 (&(&zone->lock)->rlock){-.-.}, at: rmqueue_pcplist mm/page_alloc.c:3050 [inline] 000000009bb7bae0 (&(&zone->lock)->rlock){-.-.}, at: rmqueue mm/page_alloc.c:3072 [inline] 000000009bb7bae0 (&(&zone->lock)->rlock){-.-.}, at: get_page_from_freelist+0x1bae/0x52a0 mm/page_alloc.c:3491 It appears to be a false positive in that the only way the lock ordering should be inverted is if kswapd is waking itself and the wakeup allocates debugging objects which should already be allocated if it's kswapd doing the waking. Nevertheless, the possibility exists and so it's best to avoid the problem. This patch flags a zone as needing a kswapd using the, surprisingly, unused zone flag field. The flag is read without the lock held to do the wakeup. It's possible that the flag setting context is not the same as the flag clearing context or for small races to occur. However, each race possibility is harmless and there is no visible degredation in fragmentation treatment. While zone->flag could have continued to be unused, there is potential for moving some existing fields into the flags field instead. Particularly read-mostly ones like zone->initialized and zone->contiguous. Link: http://lkml.kernel.org/r/20190103225712.GJ31517@techsingularity.net Fixes: 1c30844d2dfe ("mm: reclaim small amounts of memory when an external fragmentation event occurs") Reported-by: syzbot+93d94a001cfbce9e60e1@syzkaller.appspotmail.com Signed-off-by: Mel Gorman Acked-by: Vlastimil Babka Tested-by: Qian Cai Cc: Dmitry Vyukov Cc: Vlastimil Babka Cc: Michal Hocko Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- include/linux/mmzone.h | 6 ++++++ 1 file changed, 6 insertions(+) (limited to 'include/linux') diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h index cc4a507d7ca4..842f9189537b 100644 --- a/include/linux/mmzone.h +++ b/include/linux/mmzone.h @@ -520,6 +520,12 @@ enum pgdat_flags { PGDAT_RECLAIM_LOCKED, /* prevents concurrent reclaim */ }; +enum zone_flags { + ZONE_BOOSTED_WATERMARK, /* zone recently boosted watermarks. + * Cleared when kswapd is woken. + */ +}; + static inline unsigned long zone_managed_pages(struct zone *zone) { return (unsigned long)atomic_long_read(&zone->managed_pages); -- cgit v1.2.3