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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