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

[Michal Hocko]

On Fri 14-02-20 08:29:45, Yang Shi 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.

Looks like an omission. The readahead flag has been added later (2.6.23)
while the migration predates 2.6.17.

> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
> I didn't experience any real problem, found by visual inspection. And, this was
> discussed in thread: https://lore.kernel.org/linux-mm/185ce762-f25d-a013-6daa-8c288f1ff791@xxxxxxxxxxxxxxxxx/T/#m1977ce1de513401b7d09d6fa14fcffe849580aae
> 
>  mm/migrate.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index edf42ed..f3c492d 100644
> --- 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);
>  
>  	mem_cgroup_migrate(page, newpage);
> -- 
> 1.8.3.1

-- 
Michal Hocko
SUSE Labs