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

[张佳辰 <zhangjiachen.jaycee@bytedance.com>]

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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.

Jiachen

>
> Regards,
> Daniel
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