Re: [RFC PATCH] mm: move xa forward when run across zombie page

[Zhaoyang Huang]

On Mon, Oct 17, 2022 at 1:34 PM Zhaoyang Huang <huangzhaoyang@xxxxxxxxx> wrote:
>
> On Fri, Oct 14, 2022 at 8:12 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> >
> > On Fri, Oct 14, 2022 at 01:30:48PM +0800, zhaoyang.huang wrote:
> > > From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> > >
> > > Bellowing RCU stall is reported where kswapd traps in a live lock when shrink
> > > superblock's inode list. The direct reason is zombie page keeps staying on the
> > > xarray's slot and make the check and retry loop permanently. The root cause is unknown yet
> > > and supposed could be an xa update without synchronize_rcu etc. I would like to
> > > suggest skip this page to break the live lock as a workaround.
> >
> > No, the underlying bug should be fixed.
> >
> > >       if (!folio || xa_is_value(folio))
> > >               return folio;
> > >
> > > -     if (!folio_try_get_rcu(folio))
> > > +     if (!folio_try_get_rcu(folio)) {
> > > +             xas_advance(xas, folio->index + folio_nr_pages(folio) - 1);
> > >               goto reset;
> > > +     }
> >
> > You can't do this anyway.  To call folio_nr_pages() and to look at
> > folio->index, you must have a refcount on the page, and this is the
> > path where we failed to get the refcount.
> OK, could I move the xas like below?
>
> +     if (!folio_try_get_rcu(folio)) {
> +             xas_next_offset(xas);
>               goto reset;
> +     }
sorry, It seems the above suggestion doesn't break the loop. update it
by moving one step and goto retry instead of restarting from ROOT.
      if (!folio_try_get_rcu(folio)) {
              xas_next_offset(xas);
               goto retry;
      }