Re: [PATCH] mm: avoid blocking lock_page() in kcompactd

From: Cong Wang
Date: Mon Jan 20 2020 - 17:42:04 EST

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On Fri, Jan 10, 2020 at 1:22 AM Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, Jan 09, 2020 at 02:56:46PM -0800, Cong Wang wrote:
> > We observed kcompactd hung at __lock_page():
> >
> > INFO: task kcompactd0:57 blocked for more than 120 seconds.
> > Not tainted 4.19.56.x86_64 #1
> > "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > kcompactd0 D 0 57 2 0x80000000
> > Call Trace:
> > ? __schedule+0x236/0x860
> > schedule+0x28/0x80
> > io_schedule+0x12/0x40
> > __lock_page+0xf9/0x120
> > ? page_cache_tree_insert+0xb0/0xb0
> > ? update_pageblock_skip+0xb0/0xb0
> > migrate_pages+0x88c/0xb90
> > ? isolate_freepages_block+0x3b0/0x3b0
> > compact_zone+0x5f1/0x870
> > kcompactd_do_work+0x130/0x2c0
> > ? __switch_to_asm+0x35/0x70
> > ? __switch_to_asm+0x41/0x70
> > ? kcompactd_do_work+0x2c0/0x2c0
> > ? kcompactd+0x73/0x180
> > kcompactd+0x73/0x180
> > ? finish_wait+0x80/0x80
> > kthread+0x113/0x130
> > ? kthread_create_worker_on_cpu+0x50/0x50
> > ret_from_fork+0x35/0x40
> >
> > which faddr2line maps to:
> >
> > migrate_pages+0x88c/0xb90:
> > lock_page at include/linux/pagemap.h:483
> > (inlined by) __unmap_and_move at mm/migrate.c:1024
> > (inlined by) unmap_and_move at mm/migrate.c:1189
> > (inlined by) migrate_pages at mm/migrate.c:1419
> >
> > Sometimes kcompactd eventually got out of this situation, sometimes not.
> >
> > I think for memory compaction, it is a best effort to migrate the pages,
> > so it doesn't have to wait for I/O to complete. It is fine to call
> > trylock_page() here, which is pretty much similar to
> > buffer_migrate_lock_buffers().
> >
> > Given MIGRATE_SYNC_LIGHT is used on compaction path, just relax the
> > check for it.
> >
>
> Is this a single page being locked for a long time or multiple pages
> being locked without reaching a reschedule point?

Not sure whether it is single page or multiple pages, but I successfully
located the process locking the page (or pages), and I used perf to
capture its stack trace:

ffffffffa722aa06 shrink_inactive_list
ffffffffa722b3d7 shrink_node_memcg
ffffffffa722b85f shrink_node
ffffffffa722bc89 do_try_to_free_pages
ffffffffa722c179 try_to_free_mem_cgroup_pages
ffffffffa7298703 try_charge
ffffffffa729a886 mem_cgroup_try_charge
ffffffffa720ec03 __add_to_page_cache_locked
ffffffffa720ee3a add_to_page_cache_lru
ffffffffa7312ddb iomap_readpages_actor
ffffffffa73133f7 iomap_apply
ffffffffa73135da iomap_readpages
ffffffffa722062e read_pages
ffffffffa7220b3f __do_page_cache_readahead
ffffffffa7210554 filemap_fault
ffffffffc039e41f __xfs_filemap_fault
ffffffffa724f5e7 __do_fault
ffffffffa724c5f2 __handle_mm_fault
ffffffffa724cbc6 handle_mm_fault
ffffffffa70a313e __do_page_fault
ffffffffa7a00dfe page_fault

This process got stuck in this situation for a long time (since I sent out
this patch) without making any progress. It behaves like stuck in an infinite
loop, although the EIP still moves around within mem_cgroup_try_charge().

I also enabled trace event mm_vmscan_lru_shrink_inactive(), here is what
I collected:

<...>-455459 [003] .... 2691911.664706:
mm_vmscan_lru_shrink_inactive: nid=0 nr_scanned=1 nr_reclaimed=0
nr_dirty=0 nr_writeback=0 nr_congested=0 nr_immediate=0 nr_activat
e=0 nr_ref_keep=0 nr_unmap_fail=0 priority=0
flags=RECLAIM_WB_FILE|RECLAIM_WB_ASYNC
<...>-455459 [003] .... 2691911.664711:
mm_vmscan_lru_shrink_inactive: nid=0 nr_scanned=1 nr_reclaimed=0
nr_dirty=0 nr_writeback=0 nr_congested=0 nr_immediate=0 nr_activat
e=0 nr_ref_keep=0 nr_unmap_fail=0 priority=4
flags=RECLAIM_WB_FILE|RECLAIM_WB_ASYNC
<...>-455459 [003] .... 2691911.664714:
mm_vmscan_lru_shrink_inactive: nid=0 nr_scanned=2 nr_reclaimed=0
nr_dirty=0 nr_writeback=0 nr_congested=0 nr_immediate=0 nr_activat
e=0 nr_ref_keep=0 nr_unmap_fail=0 priority=3
flags=RECLAIM_WB_FILE|RECLAIM_WB_ASYNC
<...>-455459 [003] .... 2691911.664717:
mm_vmscan_lru_shrink_inactive: nid=0 nr_scanned=5 nr_reclaimed=0
nr_dirty=0 nr_writeback=0 nr_congested=0 nr_immediate=0 nr_activat
e=0 nr_ref_keep=0 nr_unmap_fail=0 priority=2
flags=RECLAIM_WB_FILE|RECLAIM_WB_ASYNC
<...>-455459 [003] .... 2691911.664720:
mm_vmscan_lru_shrink_inactive: nid=0 nr_scanned=5 nr_reclaimed=0
nr_dirty=0 nr_writeback=0 nr_congested=0 nr_immediate=0 nr_activat
e=0 nr_ref_keep=0 nr_unmap_fail=0 priority=1
flags=RECLAIM_WB_FILE|RECLAIM_WB_ASYNC
<...>-455459 [003] .... 2691911.664725:
mm_vmscan_lru_shrink_inactive: nid=0 nr_scanned=7 nr_reclaimed=0
nr_dirty=0 nr_writeback=0 nr_congested=0 nr_immediate=0 nr_activat
e=0 nr_ref_keep=0 nr_unmap_fail=0 priority=0
flags=RECLAIM_WB_FILE|RECLAIM_WB_ASYNC
<...>-455459 [003] .... 2691911.664730:
mm_vmscan_lru_shrink_inactive: nid=0 nr_scanned=1 nr_reclaimed=0
nr_dirty=0 nr_writeback=0 nr_congested=0 nr_immediate=0 nr_activat
e=0 nr_ref_keep=0 nr_unmap_fail=0 priority=2
flags=RECLAIM_WB_FILE|RECLAIM_WB_ASYNC
<...>-455459 [003] .... 2691911.664732:
mm_vmscan_lru_shrink_inactive: nid=0 nr_scanned=1 nr_reclaimed=0
nr_dirty=0 nr_writeback=0 nr_congested=0 nr_immediate=0 nr_activat
e=0 nr_ref_keep=0 nr_unmap_fail=0 priority=0
flags=RECLAIM_WB_FILE|RECLAIM_WB_ASYNC
<...>-455459 [003] .... 2691911.664736:
mm_vmscan_lru_shrink_inactive: nid=0 nr_scanned=1 nr_reclaimed=0
nr_dirty=0 nr_writeback=0 nr_congested=0 nr_immediate=0 nr_activat
e=0 nr_ref_keep=0 nr_unmap_fail=0 priority=4
flags=RECLAIM_WB_FILE|RECLAIM_WB_ASYNC
<...>-455459 [003] .... 2691911.664739:
mm_vmscan_lru_shrink_inactive: nid=0 nr_scanned=2 nr_reclaimed=0
nr_dirty=0 nr_writeback=0 nr_congested=0 nr_immediate=0 nr_activat
e=0 nr_ref_keep=0 nr_unmap_fail=0 priority=3
flags=RECLAIM_WB_FILE|RECLAIM_WB_ASYNC
<...>-455459 [003] .... 2691911.664744:
mm_vmscan_lru_shrink_inactive: nid=0 nr_scanned=5 nr_reclaimed=0
nr_dirty=0 nr_writeback=0 nr_congested=0 nr_immediate=0 nr_activat
e=0 nr_ref_keep=0 nr_unmap_fail=0 priority=2
flags=RECLAIM_WB_FILE|RECLAIM_WB_ASYNC
<...>-455459 [003] .... 2691911.664747:
mm_vmscan_lru_shrink_inactive: nid=0 nr_scanned=4 nr_reclaimed=0
nr_dirty=0 nr_writeback=0 nr_congested=0 nr_immediate=0 nr_activat
e=0 nr_ref_keep=0 nr_unmap_fail=0 priority=1
flags=RECLAIM_WB_FILE|RECLAIM_WB_ASYNC
<...>-455459 [003] .... 2691911.664752:
mm_vmscan_lru_shrink_inactive: nid=0 nr_scanned=12 nr_reclaimed=0
nr_dirty=0 nr_writeback=0 nr_congested=0 nr_immediate=0 nr_activa
te=0 nr_ref_keep=0 nr_unmap_fail=0 priority=0
flags=RECLAIM_WB_FILE|RECLAIM_WB_ASYNC
<...>-455459 [003] .... 2691911.664755:
mm_vmscan_lru_shrink_inactive: nid=0 nr_scanned=1 nr_reclaimed=0
nr_dirty=0 nr_writeback=0 nr_congested=0 nr_immediate=0 nr_activat
e=0 nr_ref_keep=0 nr_unmap_fail=0 priority=4
flags=RECLAIM_WB_FILE|RECLAIM_WB_ASYNC
<...>-455459 [003] .... 2691911.664761:
mm_vmscan_lru_shrink_inactive: nid=0 nr_scanned=1 nr_reclaimed=0
nr_dirty=0 nr_writeback=0 nr_congested=0 nr_immediate=0 nr_activat
e=0 nr_ref_keep=0 nr_unmap_fail=0 priority=2
flags=RECLAIM_WB_FILE|RECLAIM_WB_ASYNC
<...>-455459 [003] .... 2691911.664762:
mm_vmscan_lru_shrink_inactive: nid=0 nr_scanned=1 nr_reclaimed=0
nr_dirty=0 nr_writeback=0 nr_congested=0 nr_immediate=0 nr_activat
e=0 nr_ref_keep=0 nr_unmap_fail=0 priority=1
flags=RECLAIM_WB_FILE|RECLAIM_WB_ASYNC
<...>-455459 [003] .... 2691911.664764:
mm_vmscan_lru_shrink_inactive: nid=0 nr_scanned=1 nr_reclaimed=0
nr_dirty=0 nr_writeback=0 nr_congested=0 nr_immediate=0 nr_activat
e=0 nr_ref_keep=0 nr_unmap_fail=0 priority=0
flags=RECLAIM_WB_FILE|RECLAIM_WB_ASYNC
<...>-455459 [003] .... 2691911.664770:
mm_vmscan_lru_shrink_inactive: nid=0 nr_scanned=4 nr_reclaimed=0
nr_dirty=0 nr_writeback=0 nr_congested=0 nr_immediate=0 nr_activat
e=0 nr_ref_keep=0 nr_unmap_fail=0 priority=1
flags=RECLAIM_WB_FILE|RECLAIM_WB_ASYNC
<...>-455459 [003] .... 2691911.664777:
mm_vmscan_lru_shrink_inactive: nid=0 nr_scanned=21 nr_reclaimed=0
nr_dirty=0 nr_writeback=0 nr_congested=0 nr_immediate=0 nr_activa
te=0 nr_ref_keep=0 nr_unmap_fail=0 priority=0
flags=RECLAIM_WB_FILE|RECLAIM_WB_ASYNC
<...>-455459 [003] .... 2691911.664780:
mm_vmscan_lru_shrink_inactive: nid=0 nr_scanned=1 nr_reclaimed=0
nr_dirty=0 nr_writeback=0 nr_congested=0 nr_immediate=0 nr_activat
e=0 nr_ref_keep=0 nr_unmap_fail=0 priority=4
flags=RECLAIM_WB_FILE|RECLAIM_WB_ASYNC
<...>-455459 [003] .... 2691911.664783:
mm_vmscan_lru_shrink_inactive: nid=0 nr_scanned=2 nr_reclaimed=0
nr_dirty=0 nr_writeback=0 nr_congested=0 nr_immediate=0 nr_activat
e=0 nr_ref_keep=0 nr_unmap_fail=0 priority=3
flags=RECLAIM_WB_FILE|RECLAIM_WB_ASYNC

>
> If it's a single page being locked, it's important to identify what held
> page lock for 2 minutes because that is potentially a missing
> unlock_page. The kernel in question is old -- 4.19.56. Are there any
> other modifications to that kernel?

We only backported some networking and hardware driver patches,
not any MM change.

Please let me know if I can collect any other information you need,
before it gets rebooted.

Thanks.

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