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

[Yang Shi]

On 2/13/20 6:55 PM, Andrew Morton wrote:
> 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

The newpage may not have writeback set, migrating readahead flag should 
not depend on it.

>    	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);
> 	}
>
> ?