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
