Re: [Ocfs2-devel] [PATCH v3 7/7] gfs2: Fix mmap + page fault deadlocks for direct I/O

["Andreas Grünbacher" <andreas.gruenbacher@gmail.com>]

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Jan Kara <jack@suse.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@redhat.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.

Andreas


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

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