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

Matthew Wilcox
2020-02-14T04:07:31Z

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@linux.alibaba.com> 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);
> > }
> >
> > ?
>