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

Andrew Morton
2020-02-14T02:55:15Z

On Fri, 14 Feb 2020 08:29:45 +0800 Yang Shi <yang.shi@linux.alibaba.com> 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);
}

?