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

[Michal Hocko]

[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@xxxxxxxxxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Signed-off-by: Cong Wang <xiyou.wangcong@xxxxxxxxx>
> ---
>  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