linux-kernel - Re: [PATCH v3 7/7] gfs2: Fix mmap + page fault deadlocks for direct I/O

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Message-ID: <20210726180042.GN20621@quack2.suse.cz>
Date:   Mon, 26 Jul 2021 20:00:42 +0200
From:   Jan Kara <jack@...e.cz>
To:     Andreas Grünbacher 
        <andreas.gruenbacher@...il.com>
Cc:     Jan Kara <jack@...e.cz>, Andreas Gruenbacher <agruenba@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        cluster-devel <cluster-devel@...hat.com>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        ocfs2-devel@....oracle.com
Subject: Re: [PATCH v3 7/7] gfs2: Fix mmap + page fault deadlocks for direct
 I/O

On Mon 26-07-21 19:50:23, Andreas Grünbacher wrote:
> Jan Kara <jack@...e.cz> schrieb am Mo., 26. Juli 2021, 19:10:
> 
> > On Fri 23-07-21 22:58:40, Andreas Gruenbacher wrote:
> > > Also disable page faults during direct I/O requests and implement the
> > same kind
> > > of retry logic as in the buffered I/O case.
> > >
> > > Direct I/O requests differ from buffered I/O requests in that they use
> > > bio_iov_iter_get_pages for grabbing page references and faulting in pages
> > > instead of triggering real page faults.  Those manual page faults can be
> > > disabled with the iocb->noio flag.
> > >
> > > Signed-off-by: Andreas Gruenbacher <agruenba@...hat.com>
> > > ---
> > >  fs/gfs2/file.c | 34 +++++++++++++++++++++++++++++++++-
> > >  1 file changed, 33 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/fs/gfs2/file.c b/fs/gfs2/file.c
> > > index f66ac7f56f6d..7986f3be69d2 100644
> > > --- a/fs/gfs2/file.c
> > > +++ b/fs/gfs2/file.c
> > > @@ -782,21 +782,41 @@ static ssize_t gfs2_file_direct_read(struct kiocb
> > *iocb, struct iov_iter *to,
> > >       struct file *file = iocb->ki_filp;
> > >       struct gfs2_inode *ip = GFS2_I(file->f_mapping->host);
> > >       size_t count = iov_iter_count(to);
> > > +     size_t written = 0;
> > >       ssize_t ret;
> > >
> > > +     /*
> > > +      * In this function, we disable page faults when we're holding the
> > > +      * inode glock while doing I/O.  If a page fault occurs, we drop
> > the
> > > +      * inode glock, fault in the pages manually, and then we retry.
> > Other
> > > +      * than in gfs2_file_read_iter, iomap_dio_rw can trigger implicit
> > as
> > > +      * well as manual page faults, and we need to disable both kinds
> > > +      * separately.
> > > +      */
> > > +
> > >       if (!count)
> > >               return 0; /* skip atime */
> > >
> > >       gfs2_holder_init(ip->i_gl, LM_ST_DEFERRED, 0, gh);
> > > +retry:
> > >       ret = gfs2_glock_nq(gh);
> > >       if (ret)
> > >               goto out_uninit;
> > >
> > > +     pagefault_disable();
> >
> > Is there any use in pagefault_disable() here? iomap_dio_rw() should not
> > trigger any page faults anyway, should it?
> >
> 
> It can trigger physical page faults when reading from holes.

Aha, good point. Maybe even worth a comment at this site? Thanks for
explanation!

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR