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References: <1665725448-31439-1-git-send-email-zhaoyang.huang@unisoc.com> <Y0lSChlclGPkwTeA@casper.infradead.org>
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From:   Zhaoyang Huang <huangzhaoyang@gmail.com>
Date:   Mon, 17 Oct 2022 13:34:13 +0800
Message-ID: <CAGWkznG=_A-3A8JCJEoWXVcx+LUNH=gvXjLpZZs0cRX4dhUJfQ@mail.gmail.com>
Subject: Re: [RFC PATCH] mm: move xa forward when run across zombie page
To:     Matthew Wilcox <willy@infradead.org>
Cc:     "zhaoyang.huang" <zhaoyang.huang@unisoc.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, ke.wang@unisoc.com,
        steve.kang@unisoc.com, baocong.liu@unisoc.com,
        linux-fsdevel@vger.kernel.org
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On Fri, Oct 14, 2022 at 8:12 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Fri, Oct 14, 2022 at 01:30:48PM +0800, zhaoyang.huang wrote:
> > From: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
> >
> > 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;
+     }