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SubjectRe: [PATCH] mm: avoid blocking lock_page() in kcompactd
[CC Mel and Vlastimil]

On Thu 09-01-20 14:56:46, 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.

What does this mean exactly? Who is holding the page lock?

> 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.

The exact definition of MIGRATE_SYNC_LIGHT is a bit fuzzy but bailing
out on the page lock sounds too early to me. So far we have tried to
block on everything but the writeback which can take a lot of time.

> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: linux-mm@kvack.org
> Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
> ---
> mm/migrate.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 86873b6f38a7..df60026779d2 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1010,7 +1010,8 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
> bool is_lru = !__PageMovable(page);
>
> if (!trylock_page(page)) {
> - if (!force || mode == MIGRATE_ASYNC)
> + if (!force || mode == MIGRATE_ASYNC
> + || mode == MIGRATE_SYNC_LIGHT)
> goto out;
>
> /*
> --
> 2.21.1
>

--
Michal Hocko
SUSE Labs

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