Re: [External] Re: [PATCH] virtiofsd: add -o allow_directio|no_directio option

["Daniel P. Berrangé" <berrange@redhat.com>]

On Sat, Aug 22, 2020 at 01:51:04AM +0800, 张佳辰 wrote:
> On Fri, Aug 21, 2020 at 7:58 PM Daniel P. Berrangé <berrange@redhat.com>
> wrote:
> 
> > On Fri, Aug 21, 2020 at 11:41:26AM +0800, Jiachen Zhang wrote:
> > > Due to the commit 65da4539803373ec4eec97ffc49ee90083e56efd, the O_DIRECT
> > > open flag of guest applications will be discarded by virtiofsd. While
> > > this behavior makes it consistent with the virtio-9p scheme when guest
> > > applications using direct I/O, we no longer have any chance to bypass
> > > the host page cache.
> > >
> > > Therefore, we add a flag 'allow_directio' to lo_data. If '-o no_directio'
> > > option is added, or none of '-o no_directio' or '-o allow_directio' is
> > > added, the 'allow_directio' will be set to 0, and virtiofsd discards
> > > O_DIRECT as before. If '-o allow_directio' is added to the stariting
> > > command-line, 'allow_directio' will be set to 1, so that the O_DIRECT
> > > flags will be retained and host page cache can be bypassed.
> > >
> > > Signed-off-by: Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>
> > > ---
> > >  tools/virtiofsd/helper.c         |  4 ++++
> > >  tools/virtiofsd/passthrough_ll.c | 20 ++++++++++++++------
> > >  2 files changed, 18 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/tools/virtiofsd/helper.c b/tools/virtiofsd/helper.c
> > > index 3105b6c23a..534ff52c64 100644
> > > --- a/tools/virtiofsd/helper.c
> > > +++ b/tools/virtiofsd/helper.c
> > > @@ -180,6 +180,10 @@ void fuse_cmdline_help(void)
> > >             "                               (0 leaves rlimit
> > unchanged)\n"
> > >             "                               default: min(1000000,
> > fs.file-max - 16384)\n"
> > >             "                                        if the current
> > rlimit is lower\n"
> > > +           "    -o allow_directio|no_directio\n"
> > > +           "                               retain/discard O_DIRECT
> > flags passed down\n"
> > > +           "                               to virtiofsd from guest
> > applications.\n"
> > > +           "                               default: no_directio\n"
> > >             );
> >
> > The standard naming convention from existing options is to use
> > $OPTNAME and no_$OPTNAME.
> >
> > IOW, don't use the "allow_" prefix. The options should be just
> > "directio" and "no_directio"
> >
> > Thanks, Daniel. I did consider using "directio" instead of "allow_directio"
> before I send out this patch. Although "-o directio" makes it consistent
> with other option names, it may confuse the users of virtiofsd.
> Because currently, virtiofsd will not add an O_DIRECT to the open flag,
> it will just retain or discard the O_DIRECT added by guest applications.
> But "-o direct" may make the users think that virtiofsd will do direct IO
> all
> the time.

Then -o allow_direct_io   and  -o no_allow_direct_io


Regards,
Daniel
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