On 11/16/18 at 10:14am, Michal Hocko wrote:
> Could you try to apply this debugging patch on top please? It will dump
> stack trace for each reference count elevation for one page that fails
> to migrate after multiple passes.

Have got a dmesg with below patch applied, please check the attachment.
Thanks.

> 
> diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
> index 14d14beb1f7f..b64ebf253381 100644
> --- a/include/linux/page_ref.h
> +++ b/include/linux/page_ref.h
> @@ -72,9 +72,12 @@ static inline int page_count(struct page *page)
>  	return atomic_read(&compound_head(page)->_refcount);
>  }
>  
> +struct page *page_to_track;
>  static inline void set_page_count(struct page *page, int v)
>  {
>  	atomic_set(&page->_refcount, v);
> +	if (page == page_to_track)
> +		dump_stack();
>  	if (page_ref_tracepoint_active(__tracepoint_page_ref_set))
>  		__page_ref_set(page, v);
>  }
> @@ -91,6 +94,8 @@ static inline void init_page_count(struct page *page)
>  static inline void page_ref_add(struct page *page, int nr)
>  {
>  	atomic_add(nr, &page->_refcount);
> +	if (page == page_to_track)
> +		dump_stack();
>  	if (page_ref_tracepoint_active(__tracepoint_page_ref_mod))
>  		__page_ref_mod(page, nr);
>  }
> @@ -105,6 +110,8 @@ static inline void page_ref_sub(struct page *page, int nr)
>  static inline void page_ref_inc(struct page *page)
>  {
>  	atomic_inc(&page->_refcount);
> +	if (page == page_to_track)
> +		dump_stack();
>  	if (page_ref_tracepoint_active(__tracepoint_page_ref_mod))
>  		__page_ref_mod(page, 1);
>  }
> @@ -129,6 +136,8 @@ static inline int page_ref_inc_return(struct page *page)
>  {
>  	int ret = atomic_inc_return(&page->_refcount);
>  
> +	if (page == page_to_track)
> +		dump_stack();
>  	if (page_ref_tracepoint_active(__tracepoint_page_ref_mod_and_return))
>  		__page_ref_mod_and_return(page, 1, ret);
>  	return ret;
> @@ -156,6 +165,8 @@ static inline int page_ref_add_unless(struct page *page, int nr, int u)
>  {
>  	int ret = atomic_add_unless(&page->_refcount, nr, u);
>  
> +	if (page == page_to_track)
> +		dump_stack();
>  	if (page_ref_tracepoint_active(__tracepoint_page_ref_mod_unless))
>  		__page_ref_mod_unless(page, nr, ret);
>  	return ret;
> diff --git a/mm/migrate.c b/mm/migrate.c
> index f7e4bfdc13b7..9b2e395a3d68 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1338,6 +1338,8 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
>  	return rc;
>  }
>  
> +struct page *page_to_track;
> +
>  /*
>   * migrate_pages - migrate the pages specified in a list, to the free pages
>   *		   supplied as the target for the page migration
> @@ -1375,6 +1377,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  	if (!swapwrite)
>  		current->flags |= PF_SWAPWRITE;
>  
> +	page_to_track = NULL;
>  	for(pass = 0; pass < 10 && retry; pass++) {
>  		retry = 0;
>  
> @@ -1417,6 +1420,8 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  				goto out;
>  			case -EAGAIN:
>  				retry++;
> +				if (pass > 1 && !page_to_track)
> +					page_to_track = page;
>  				break;
>  			case MIGRATEPAGE_SUCCESS:
>  				nr_succeeded++;
> -- 
> Michal Hocko
> SUSE Labs