Re: [PATCH] mm/page-writeback: introduce tracepoint for wait_on_page_writeback

[Yafang Shao <laoar.shao@gmail.com>]

On Sat, Apr 27, 2019 at 2:25 AM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Fri, 26 Apr 2019 18:26:42 +0800 Yafang Shao <laoar.shao@gmail.com> wrote:
>
> > Recently there're some hungtasks on our server due to
> > wait_on_page_writeback, and we want to know the details of this
> > PG_writeback, i.e. this page is writing back to which device.
> > But it is not so convenient to get the details.
> >
> > I think it would be better to introduce a tracepoint for diagnosing
> > the writeback details.
>
> Fair enough, I guess.
>
> > --- a/include/linux/pagemap.h
> > +++ b/include/linux/pagemap.h
> > @@ -537,15 +537,7 @@ static inline int wait_on_page_locked_killable(struct page *page)
> >
> >  extern void put_and_wait_on_page_locked(struct page *page);
> >
> > -/*
> > - * Wait for a page to complete writeback
> > - */
> > -static inline void wait_on_page_writeback(struct page *page)
> > -{
> > -     if (PageWriteback(page))
> > -             wait_on_page_bit(page, PG_writeback);
> > -}
> > -
> > +void wait_on_page_writeback(struct page *page);
> >  extern void end_page_writeback(struct page *page);
> >  void wait_for_stable_page(struct page *page);
> >
> > ...
> >
> > +/*
> > + * Wait for a page to complete writeback
> > + */
> > +void wait_on_page_writeback(struct page *page)
> > +{
> > +     if (PageWriteback(page)) {
> > +             trace_wait_on_page_writeback(page, page_mapping(page));
> > +             wait_on_page_bit(page, PG_writeback);
> > +     }
> > +}
> > +EXPORT_SYMBOL_GPL(wait_on_page_writeback);
>
> But this is a stealth change to the wait_on_page_writeback() licensing.
> I will get sad emails from developers of accidentally-broken
> out-of-tree filesystems.
>
> We can discuss changing the licensing, but this isn't the way to do it!
>

Got it.
Thanks for your explatation.

Thanks
Yafang