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

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

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"


Regards,
Daniel
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