Re: [Virtio-fs] [PATCH v7 5/6] virtiofsd: implement xflag based dax policy

[Vivek Goyal <vgoyal@redhat.com>]

On Tue, Nov 02, 2021 at 01:56:45PM +0800, Jeffle Xu wrote:
> The per inode DAX feature in ext4/xfs uses the persistent inode flag,
> i.e. FS_DAX_FL/FS_XFLAG_DAX, to indicate if DAX shall be enabled for
> this file or not when filesystem is mounted in per inode DAX mode.
> 
> To keep compatible with this feature in ext4/xfs, virtiofs also supports
> enabling DAX depending on the persistent inode flag, which may be
> set/cleared by users inside guest or admin on host.
> 
> This policy is used when '-o dax=inode' option is specified for
> virtiofsd.
> 
> Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
> ---
>  tools/virtiofsd/passthrough_ll.c | 33 ++++++++++++++++++++++++++++++++
>  1 file changed, 33 insertions(+)
> 
> diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
> index 999c906da2..dac5063594 100644
> --- a/tools/virtiofsd/passthrough_ll.c
> +++ b/tools/virtiofsd/passthrough_ll.c
> @@ -1026,6 +1026,35 @@ static int do_statx(struct lo_data *lo, int dirfd, const char *pathname,
>      return 0;
>  }
>  
> +static bool lo_should_enable_dax(struct lo_data *lo, struct lo_inode *inode,
> +                                 struct fuse_entry_param *e)
> +{
> +    if (lo->dax == INODE_DAX_NONE || !S_ISREG(e->attr.st_mode)) {
> +        return false;
> +    }
> +
> +    if (lo->dax == INODE_DAX_INODE) {
> +        int res, fd;
> +        int ret = false;
> +        unsigned int attr;
> +
> +        fd = lo_inode_open(lo, inode, O_RDONLY);
> +        if (fd == -1) {
> +            return false;
> +        }
> +
> +        res = ioctl(fd, FS_IOC_GETFLAGS, &attr);
> +        if (!res && (attr & FS_DAX_FL)) {
> +            ret = true;
> +        }
> +
> +        close(fd);
> +        return ret;
> +    }
> +
> +    return false;
> +}
> +
>  /*
>   * Increments nlookup on the inode on success. unref_inode_lolocked() must be
>   * called eventually to decrement nlookup again. If inodep is non-NULL, the
> @@ -1116,6 +1145,10 @@ static int lo_do_lookup(fuse_req_t req, fuse_ino_t parent, const char *name,
>      }
>      e->ino = inode->fuse_ino;
>  
> +    if (lo_should_enable_dax(lo, inode, e)) {
> +        e->attr_flags |= FUSE_ATTR_DAX;
> +    }
> +

If we were too strict then we should probably add a
lo_should_enable_dax() call in lo_getattr() as well?

I guess for now it is ok. Overhead of checking fs_xflag_dax seems
enough and not many might care about immediate chnage of dax
status in guest.

Vivek

>      /* Transfer ownership of inode pointer to caller or drop it */
>      if (inodep) {
>          *inodep = inode;
> -- 
> 2.27.0
>