Re: [Virtio-fs] [PATCH v6 4/7] fuse: negotiate per-file DAX in FUSE_INIT

[Vivek Goyal <vgoyal@redhat.com>]

On Wed, Oct 20, 2021 at 11:10:30AM +0800, JeffleXu wrote:
> 
> 
> On 10/18/21 10:30 PM, Vivek Goyal wrote:
> > On Mon, Oct 11, 2021 at 11:00:49AM +0800, Jeffle Xu wrote:
> >> Among the FUSE_INIT phase, client shall advertise per-file DAX if it's
> >> mounted with "-o dax=inode". Then server is aware that client is in
> >> per-file DAX mode, and will construct per-inode DAX attribute
> >> accordingly.
> >>
> >> Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
> >> ---
> >>  fs/fuse/inode.c | 2 ++
> >>  1 file changed, 2 insertions(+)
> >>
> >> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> >> index b4b41683e97e..f4ad99e2415b 100644
> >> --- a/fs/fuse/inode.c
> >> +++ b/fs/fuse/inode.c
> >> @@ -1203,6 +1203,8 @@ void fuse_send_init(struct fuse_mount *fm)
> >>  #ifdef CONFIG_FUSE_DAX
> >>  	if (fm->fc->dax)
> >>  		ia->in.flags |= FUSE_MAP_ALIGNMENT;
> >> +	if (fm->fc->dax_mode == FUSE_DAX_INODE)
> >> +		ia->in.flags |= FUSE_PERFILE_DAX;
> > 
> > Are you not keeping track of server's response whether server supports
> > per inode dax or not. Client might be new and server might be old and
> > server might not support per inode dax. In that case, we probably 
> > should error out if user mounted with "-o dax=inode".
> > 
> 
> Yes, if guest virtiofs is mounted with '-o dax=inode' while virtiofsd is
> old and doesn't support per inode dax, then guest virtiofs will never
> receive FUSE_ATTR_DAX and actually behaves as '-o dax=never'. So the
> whole system works in this case, though the behavior may be beyond the
> expectation of users ....
> 
> If the behavior really matters, I could change the behavior and fail
> directly if virtiofsd doesn't advertise supporting per inode DAX.

I think it probably is better to error out if client asked for per-inode
DAX and server does not support it. 

Vivek
> 
> -- 
> Thanks,
> Jeffle
>