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

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Message-ID: <CAGWkznEdtGPPZkHrq6Y_+XLL37w12aC8XN8R_Q-vhq48rFhkSA@mail.gmail.com>
Date:   Tue, 18 Oct 2022 10:52:19 +0800
From:   Zhaoyang Huang <huangzhaoyang@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     "zhaoyang.huang" <zhaoyang.huang@...soc.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, ke.wang@...soc.com,
        steve.kang@...soc.com, baocong.liu@...soc.com,
        linux-fsdevel@...r.kernel.org
Subject: Re: [RFC PATCH] mm: move xa forward when run across zombie page

On Mon, Oct 17, 2022 at 11:55 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Mon, Oct 17, 2022 at 01:34:13PM +0800, Zhaoyang Huang wrote:
> > On Fri, Oct 14, 2022 at 8:12 PM Matthew Wilcox <willy@...radead.org> wrote:
> > >
> > > On Fri, Oct 14, 2022 at 01:30:48PM +0800, zhaoyang.huang wrote:
> > > > From: Zhaoyang Huang <zhaoyang.huang@...soc.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.
>
>     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Understand. IMHO, find_get_entry actruely works as an open API dealing
with different kinds of address_spaces page cache, which requires high
robustness to deal with any corner cases. Take the current problem as
example, the inode with fault page(refcount=0) could remain on the
sb's list without live lock problem.
>
> > OK, could I move the xas like below?
> >
> > +     if (!folio_try_get_rcu(folio)) {
> > +             xas_next_offset(xas);
> >               goto reset;
> > +     }