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

Michal Hocko
2020-02-14T08:34:48Z

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@infradead.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> 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@linux.alibaba.com/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