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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Liu Bo (bo.liu@linux.alibaba.com) wrote:
> 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@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.

Right; that should be O_WRONLY;  
do_setupmapping does:

        genflags = 0;
        genflags |= (arg->flags & FUSE_SETUPMAPPING_FLAG_WRITE) ? O_WRONLY : 0;

to convert from the fuse definition on the wire to the O_ notation
before calling the filesstem code.

Dave

> 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);
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK