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

[Matthew Wilcox]

On Thu, Feb 13, 2020 at 07:58:40PM -0800, Yang Shi wrote:
> 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.

Indeed, if the page has writeback set, then the page does not have the
readahead flag set; it has the reclaim flag set.  The original patch is
correct, afaict.

> >    	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);
> > 	}
> > 
> > ?
>