[Virtio-fs] [PATCH v4 2/2] virtiofsd: use fuse_buf_writev to replace fuse_buf_write for better performance

[Stefan Hajnoczi]

On Thu, Aug 15, 2019 at 09:11:49AM +0800, piaojun wrote:
> Hi Stefan,
> 
> On 2019/8/14 20:52, Stefan Hajnoczi wrote:
> > On Fri, Aug 09, 2019 at 08:53:20AM +0800, piaojun wrote:
> >> diff --git a/contrib/virtiofsd/buffer.c b/contrib/virtiofsd/buffer.c
> >> index cec762f..d127c45 100644
> >> --- a/contrib/virtiofsd/buffer.c
> >> +++ b/contrib/virtiofsd/buffer.c
> >> @@ -318,11 +318,22 @@ static int fuse_bufvec_advance(struct fuse_bufvec *bufv, size_t len)
> >>  ssize_t fuse_buf_copy(fuse_req_t req, struct fuse_bufvec *dstv, struct fuse_bufvec *srcv,
> >>  		      enum fuse_buf_copy_flags flags)
> >>  {
> >> +	size_t i;
> >>  	size_t copied = 0;
> >>
> >>  	if (dstv == srcv)
> >>  		return fuse_buf_size(dstv);
> >>
> >> +	/* use writev to improve bandwidth when all the
> >> +	 * src buffers already mapped by the daemon
> >> +	 * process */
> > 
> > Some more checks to make this generic:
> > 
> >> +	for (i = 0; i < srcv->count; i++) {
> >> +		if (srcv->buf[i].flags & FUSE_BUF_PHYS_ADDR)
> >> +			break;
> > 
> >                 if (srcv->buf[i].flags & FUSE_BUF_IS_FD)
> > 		        break;
> > 
> >> +	}
> >> +	if (i == srcv->count)
> > 
> > Please verify the preconditions for the destination buffer too:
> > 
> >   && dstv->count == 1 && dstv->idx == 0 && dstv->off == 0 &&
> >   (dstv->buf[0].flags & FUSE_BUF_IS_FD)
> 
> Thanks for your comments, and I prefer adding checker just for
> FUSE_BUF_IS_FD as fuse_buf_writev() does not care much about the other
> parts. And I personally think so many preconditions will make
> fuse_buf_writev() so specific.

I think these preconditions are necessary.  This function won't do the
right thing if dstv has multiple elements or has been previously
modified (idx/off).

Can you explain why these cases will never happen or why the code is
correct under these cases?

Stefan
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