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

[JeffleXu]

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 at 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.

-- 
Thanks,
Jeffle