[Virtio-fs] [PATCH] virtiofsd: During setup mapping, open file O_RDWR only if needed

[Liu Bo]

On Wed, Jul 24, 2019 at 05:10:24PM -0400, Vivek Goyal wrote:
> As of now we always open file O_RDWR (even if writable mappings are not
> required). This leads to copy up of file if file is backed by overlayfs
> and hence nullying advantages of overlayfs.
> 
> So open file O_RDONLY if writable mappings are not required. Open O_RDWR
> if writable mappings are needed.
> 
> Signed-off-by: Vivek Goyal <vgoyal at redhat.com>
> ---
>  contrib/virtiofsd/passthrough_ll.c |    9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> Index: qemu/contrib/virtiofsd/passthrough_ll.c
> ===================================================================
> --- qemu.orig/contrib/virtiofsd/passthrough_ll.c	2019-07-24 16:31:07.014871768 -0400
> +++ qemu/contrib/virtiofsd/passthrough_ll.c	2019-07-24 16:32:10.064412722 -0400
> @@ -2197,15 +2197,15 @@ static void lo_setupmapping(fuse_req_t r
>          VhostUserFSSlaveMsg msg = { 0 };
>          uint64_t vhu_flags;
>  	char *buf;
> +	bool writable = flags & FUSE_SETUPMAPPING_FLAG_WRITE;

typo? flags is set to O_WRONLY or 0 in do_lookup, although this may work as same.

thanks,
-liubo
>  
>  	if (lo_debug(req))
>  		fuse_debug("lo_setupmapping(ino=%" PRIu64 ", fi=0x%p)\n", ino,
>  			   (void *)fi);
>  
>          vhu_flags = VHOST_USER_FS_FLAG_MAP_R;
> -        if (flags & O_WRONLY) {
> +	if (writable)
>                  vhu_flags |= VHOST_USER_FS_FLAG_MAP_W;
> -        }
>  
>  	msg.fd_offset[0] = foffset;
>  	msg.len[0] = len;
> @@ -2223,7 +2223,10 @@ static void lo_setupmapping(fuse_req_t r
>  		 * TODO: O_RDWR might not be allowed if file is read only or
>  		 * write only. Fix it.
>  		 */
> -		fd = openat(lo->proc_self_fd, buf, O_RDWR);
> +		if (writable)
> +			fd = openat(lo->proc_self_fd, buf, O_RDWR);
> +		else
> +			fd = openat(lo->proc_self_fd, buf, O_RDONLY);
>  		free(buf);
>  		if (fd == -1)
>  			return (void) fuse_reply_err(req, errno);