linux-ext4 - Re: [Cluster-devel] [PATCH v11 16/25] fs: Convert mpage_readpages to mpage

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Date:   Thu, 18 Jun 2020 18:40:15 +0200
From:   Andreas Gruenbacher <agruenba@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Andreas Grünbacher <andreas.gruenbacher@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        Junxiao Bi <junxiao.bi@...cle.com>,
        William Kucharski <william.kucharski@...cle.com>,
        Joseph Qi <joseph.qi@...ux.alibaba.com>,
        John Hubbard <jhubbard@...dia.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-f2fs-devel@...ts.sourceforge.net,
        cluster-devel <cluster-devel@...hat.com>,
        Linux-MM <linux-mm@...ck.org>, ocfs2-devel@....oracle.com,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        linux-erofs@...ts.ozlabs.org, Christoph Hellwig <hch@....de>,
        linux-btrfs@...r.kernel.org,
        Steven Whitehouse <swhiteho@...hat.com>,
        Bob Peterson <rpeterso@...hat.com>
Subject: Re: [Cluster-devel] [PATCH v11 16/25] fs: Convert mpage_readpages to mpage_readahead

On Thu, Jun 18, 2020 at 5:03 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Thu, Jun 18, 2020 at 02:46:03PM +0200, Andreas Gruenbacher wrote:
> > On Wed, Jun 17, 2020 at 4:22 AM Matthew Wilcox <willy@...radead.org> wrote:
> > > On Wed, Jun 17, 2020 at 02:57:14AM +0200, Andreas GrÃ¼nbacher wrote:
> > > > Right, the approach from the following thread might fix this:
> > > >
> > > > https://lore.kernel.org/linux-fsdevel/20191122235324.17245-1-agruenba@redhat.com/T/#t
> > >
> > > In general, I think this is a sound approach.
> > >
> > > Specifically, I think FAULT_FLAG_CACHED can go away.  map_pages()
> > > will bring in the pages which are in the page cache, so when we get to
> > > gfs2_fault(), we know there's a reason to acquire the glock.
> >
> > We'd still be grabbing a glock while holding a dependent page lock.
> > Another process could be holding the glock and could try to grab the
> > same page lock (i.e., a concurrent writer), leading to the same kind
> > of deadlock.
>
> What I'm saying is that gfs2_fault should just be:
>
> +static vm_fault_t gfs2_fault(struct vm_fault *vmf)
> +{
> +       struct inode *inode = file_inode(vmf->vma->vm_file);
> +       struct gfs2_inode *ip = GFS2_I(inode);
> +       struct gfs2_holder gh;
> +       vm_fault_t ret;
> +       int err;
> +
> +       gfs2_holder_init(ip->i_gl, LM_ST_SHARED, 0, &gh);
> +       err = gfs2_glock_nq(&gh);
> +       if (err) {
> +               ret = block_page_mkwrite_return(err);
> +               goto out_uninit;
> +       }
> +       ret = filemap_fault(vmf);
> +       gfs2_glock_dq(&gh);
> +out_uninit:
> +       gfs2_holder_uninit(&gh);
> +       return ret;
> +}
>
> because by the time gfs2_fault() is called, map_pages() has already been
> called and has failed to insert the necessary page, so we should just
> acquire the glock now instead of trying again to look for the page in
> the page cache.

Okay, that's great.

Thanks,
Andreas