Re: [Virtio-fs] [PATCH v7 5/7] fuse: negotiate per inode DAX in FUSE_INIT

[Vivek Goyal <vgoyal@redhat.com>]

On Tue, Nov 02, 2021 at 01:26:02PM +0800, Jeffle Xu wrote:
> Among the FUSE_INIT phase, client shall advertise per inode DAX if it's
> mounted with "dax=inode". Then server is aware that client is in per
> inode DAX mode, and will construct per-inode DAX attribute accordingly.
> 
> Server shall also advertise support for per inode DAX. If server doesn't
> support it while client is mounted with "dax=inode", client will
> silently fallback to "dax=never" since "dax=inode" is advisory only.
> 
> Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
> ---
>  fs/fuse/dax.c    |  2 +-
>  fs/fuse/fuse_i.h |  3 +++
>  fs/fuse/inode.c  | 16 +++++++++++++---
>  3 files changed, 17 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/fuse/dax.c b/fs/fuse/dax.c
> index 8a328fb20dcb..c8ee601b94b8 100644
> --- a/fs/fuse/dax.c
> +++ b/fs/fuse/dax.c
> @@ -1350,7 +1350,7 @@ static bool fuse_should_enable_dax(struct inode *inode, unsigned int flags)
>  		return true;
>  
>  	/* dax_mode is FUSE_DAX_INODE or FUSE_DAX_NONE */
> -	return flags & FUSE_ATTR_DAX;
> +	return fc->inode_dax && (flags & FUSE_ATTR_DAX);
>  }
>  
>  void fuse_dax_inode_init(struct inode *inode, unsigned int flags)
> diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> index 055b39430540..58e54b5a4d65 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -777,6 +777,9 @@ struct fuse_conn {
>  	/* Propagate syncfs() to server */
>  	unsigned int sync_fs:1;
>  
> +	/* Does the filesystem support per inode DAX? */
> +	unsigned int inode_dax:1;
> +
>  	/** The number of requests waiting for completion */
>  	atomic_t num_waiting;
>  
> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> index acba14002d04..0512d8cb36c3 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -1136,11 +1136,19 @@ static void process_init_reply(struct fuse_mount *fm, struct fuse_args *args,
>  					min_t(unsigned int, fc->max_pages_limit,
>  					max_t(unsigned int, arg->max_pages, 1));
>  			}
> -			if (IS_ENABLED(CONFIG_FUSE_DAX) &&
> -			    arg->flags & FUSE_MAP_ALIGNMENT &&
> +#ifdef CONFIG_FUSE_DAX
> +			if ((arg->flags & FUSE_HAS_INODE_DAX) &&
> +			    fuse_is_inode_dax_mode(fc->dax_mode)) {

Why do we call fuse_is_inode_dax_mode() here? While sending INIT request
we set FUSE_HAS_INODE_DAX only if fuse_is_inode_dax_mode() is true. So
we should not have to call it again when server replies.?

> +				fc->inode_dax = 1;
> +			}
> +			if (arg->flags & FUSE_MAP_ALIGNMENT &&
>  			    !fuse_dax_check_alignment(fc, arg->map_alignment)) {
> -				ok = false;
> +				if (fuse_is_inode_dax_mode(fc->dax_mode))
> +					fc->inode_dax = 0;

If mapping alignment is not right, I guess we can fail (even in case
of dax=inode). In this case client wants per dax inode, server supports
it but alignment is wrong. I think that should be an error and user should
fix it. IMHO, just leave this code path in place and we will error out.

Thanks
Vivek

> +				else
> +					ok = false;
>  			}
> +#endif
>  			if (arg->flags & FUSE_HANDLE_KILLPRIV_V2) {
>  				fc->handle_killpriv_v2 = 1;
>  				fm->sb->s_flags |= SB_NOSEC;
> @@ -1194,6 +1202,8 @@ void fuse_send_init(struct fuse_mount *fm)
>  #ifdef CONFIG_FUSE_DAX
>  	if (fm->fc->dax)
>  		ia->in.flags |= FUSE_MAP_ALIGNMENT;
> +	if (fuse_is_inode_dax_mode(fm->fc->dax_mode))
> +		ia->in.flags |= FUSE_HAS_INODE_DAX;
>  #endif
>  	if (fm->fc->auto_submounts)
>  		ia->in.flags |= FUSE_SUBMOUNTS;
> -- 
> 2.27.0
>