On 29 Dec 2022 22:01:42 +0000 Matthew Wilcox <willy@xxxxxxxxxxxxx> > On Thu, Dec 29, 2022 at 01:48:42AM -0800, syzbot wrote: > > INFO: task kcompactd1:32 blocked for more than 143 seconds. > > Not tainted 6.1.0-syzkaller-14594-g72a85e2b0a1e #0 > > "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. > > task:kcompactd1 state:D stack:26360 pid:32 ppid:2 flags:0x00004000 > > Call Trace: > > <TASK> > > context_switch kernel/sched/core.c:5244 [inline] > > __schedule+0x9d1/0xe40 kernel/sched/core.c:6555 > > schedule+0xcb/0x190 kernel/sched/core.c:6631 > > io_schedule+0x83/0x100 kernel/sched/core.c:8811 > > folio_wait_bit_common+0x8ca/0x1390 mm/filemap.c:1297 > > folio_lock include/linux/pagemap.h:938 [inline] > > __unmap_and_move+0x835/0x12a0 mm/migrate.c:1040 > > unmap_and_move+0x28f/0xd80 mm/migrate.c:1194 > > migrate_pages+0x50f/0x14d0 mm/migrate.c:1477 > > compact_zone+0x2893/0x37a0 mm/compaction.c:2413 > > proactive_compact_node mm/compaction.c:2665 [inline] > > kcompactd+0x1b46/0x2750 mm/compaction.c:2975 > > OK, so kcompactd is trying to compact a zone, has called folio_lock() > and whoever has the folio locked has had it locked for 143 seconds. > That seems like quite a long time. Probably it is locked waiting > for I/O. > > > NMI backtrace for cpu 1 > [...] > > lock_release+0x81/0x870 kernel/locking/lockdep.c:5679 > > rcu_read_unlock include/linux/rcupdate.h:797 [inline] > > folio_evictable+0x1df/0x2d0 mm/internal.h:140 > > move_folios_to_lru+0x324/0x25c0 mm/vmscan.c:2413 > > shrink_inactive_list+0x60b/0xca0 mm/vmscan.c:2529 > > shrink_list mm/vmscan.c:2767 [inline] > > shrink_lruvec+0x449/0xc50 mm/vmscan.c:5951 > > shrink_node_memcgs+0x35c/0x780 mm/vmscan.c:6138 > > shrink_node+0x299/0x1050 mm/vmscan.c:6169 > > shrink_zones+0x4fb/0xc40 mm/vmscan.c:6407 > > do_try_to_free_pages+0x215/0xcd0 mm/vmscan.c:6469 > > try_to_free_pages+0x3e8/0xc60 mm/vmscan.c:6704 > > __perform_reclaim mm/page_alloc.c:4750 [inline] > > __alloc_pages_direct_reclaim mm/page_alloc.c:4772 [inline] > > __alloc_pages_slowpath+0xd5c/0x2120 mm/page_alloc.c:5178 > > __alloc_pages+0x3d4/0x560 mm/page_alloc.c:5562 > > folio_alloc+0x1a/0x50 mm/mempolicy.c:2296 > > filemap_alloc_folio+0xca/0x2c0 mm/filemap.c:972 > > page_cache_ra_unbounded+0x212/0x820 mm/readahead.c:248 > > do_sync_mmap_readahead+0x786/0x950 mm/filemap.c:3062 > > filemap_fault+0x38d/0x1060 mm/filemap.c:3154 > > So dhcpd has taken a page fault, missed in the page cache, called > readahead, is presumably partway through the readahead (ie has folios > locked in the page cache, not uptodate and I/O hasn't been submitted > on them). It's trying to allocate pages, but has fallen into reclaim. > It's trying to shrink the inactive list at this point, but is not > having much luck. For one thing, it's a GFP_NOFS allocation. So > it was probably the one who woke kcompactd. In wakeup_kswapd(), wake up kcompactd instead of kswapd because of highly fragmented but balanced node. /* * There may be plenty of free memory available, but it's too * fragmented for high-order allocations. Wake up kcompactd * and rely on compaction_suitable() to determine if it's * needed. If it fails, it will defer subsequent attempts to * ratelimit its work. */ if (!(gfp_flags & __GFP_DIRECT_RECLAIM)) wakeup_kcompactd(pgdat, order, highest_zoneidx); > > Should readahead be trying less hard to allocate memory? It's already > using __GFP_NORETRY. Given blocked kcompactd, enough pages have been reclaimed by kswapd before waking up kcompactd, and order0 allocations should be satisfied.
