Re: [PATCH] mm: migrate.c: migrate PG_readahead flag

[Andrew Morton]

On Fri, 14 Feb 2020 08:29:45 +0800 Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> wrote:

> Currently migration code doesn't migrate PG_readahead flag.
> Theoretically this would incur slight performance loss as the
> application might have to ramp its readahead back up again.  Even though
> such problem happens, it might be hidden by something else since
> migration is typically triggered by compaction and NUMA balancing, any
> of which should be more noticeable.
> 
> Migrate the flag after end_page_writeback() since it may clear
> PG_reclaim flag, which is the same bit as PG_readahead, for the new
> page.
> 
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -647,6 +647,14 @@ void migrate_page_states(struct page *newpage, struct page *page)
>  	if (PageWriteback(newpage))
>  		end_page_writeback(newpage);
>  
> +	/*
> +	 * PG_readahead share the same bit with PG_reclaim, the above
> +	 * end_page_writeback() may clear PG_readahead mistakenly, so set
> +	 * the bit after that.
> +	 */
> +	if (PageReadahead(page))
> +		SetPageReadahead(newpage);
> +
>  	copy_page_owner(page, newpage);
>  

Why not

  	if (PageWriteback(newpage)) {
  		end_page_writeback(newpage);
		/*
		 * PG_readahead share the same bit with PG_reclaim, the above
		 * end_page_writeback() may clear PG_readahead mistakenly, so
		 * set the bit after that.
		 */
		if (PageReadahead(page))
			SetPageReadahead(newpage);
	}

?