[Virtio-fs] [PATCH v3 1/2] virtiofsd: add definition of fuse_buf_writev()

[Stefan Hajnoczi]

On Thu, Aug 08, 2019 at 10:28:26PM +0800, piaojun wrote:
> On 2019/8/8 17:38, Stefan Hajnoczi wrote:
> > On Thu, Aug 08, 2019 at 02:15:27PM +0800, piaojun wrote:
> >> +{
> >> +	ssize_t res, i, buf_index, iovcnt;
> >> +	struct iovec * iov;
> >> +	int fd = out_buf->buf[0].fd;
> >> +	off_t pos = out_buf->buf[0].pos;
> > 
> > A struct fuse_bufvec may have multiple elements but this function
> > assumes it only has 1.  Please use struct fuse_buf *out_buf instead.
> > This way it's clear that only 1 fuse_buf will be written.
> 
> Do you mean that assign *out_buf with &out_buf->buf[0] in
> fuse_buf_writev() and then operate out_buf in the following process?

Since this function assumes that out_buf is only a single struct
fuse_buf, please change the argument to struct fuse_buf instead of
struct fuse_bufvec.  This way it's clear what the function expects.

> > 
> >> +
> >> +	if (in_buf->count > 2)
> >> +		iovcnt = in_buf->count - 1;
> >> +	else
> >> +		iovcnt = 1;
> >> +
> >> +	iov = calloc(iovcnt, sizeof(struct iovec));
> >> +	if (!iov)
> >> +		return -ENOMEM;
> >> +
> >> +	for (i = 0, buf_index = 1; i < iovcnt; i++, buf_index++) {
> >> +		iov[i].iov_base = in_buf->buf[buf_index].mem;
> >> +		iov[i].iov_len = in_buf->buf[buf_index].size;
> >> +	}
> > 
> > Why is in_buf->buf[0] is skipped?
> 
> This part of code seems a little complex and in_buf->buf[0].count just
> tricks me. Maybe I need handle two situations:
> 1. Iterate from in_buf->buf[0].mem when in_buf->buf[0].count is 1;
> 2. buf[0].size is set 0 in do_write_buf() when in_buf->buf[0].count > 1,
> and it's better to skip it. Perhaps it's also ok to iterate from buf[0],
> as writev skip the iov with zero len. This looks like a hack.

I would perform writev() with all of in_buf's buffers, even if some of
them have 0 length.  That way this function does not make any
assumptions about in_buf's layout.

The only requirement is that in_buf buffers are all memory buffers (not
file descriptors).
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